I had a partial case head separation using a 1.8oz buffer

[DeNinny]

I’m not following those numbers. I’m not sure what you’re saying, so it might just be me.

The deeper the bolt face, the longer the headspace.

Grendel bolts should be no deeper than .136”, but can be .003” shallower (.133”). Chambering will be more difficult and not repeatable under some conditions if you go less than .133”.

Chambering will be easier with deeper than .136” (out of spec), but then the case web won’t be supported and you can have things like this.

Most bolts I’ve measured were either .136” or .135”, but I do recall one that was .137” that probably shouldn’t have left the vendor.

Once you start exceeding the dimensions, tolerance stacking can occur and create unsafe headspace.

I am aligning to your thinking. Basically I was evaluating the difference in how much a round, with an out of spec deep bolt and with excessive headspace, would add to the unsupported side wall area. 0.138" was your out of spec condition, and 0.133" was your in spec condition. This means at least 0.005" of case depth is being exposed to unsupported wall area, in addition to what is already exposed by the feed cone. I hope this makes sense now.

And then you could even add the headspace excess to it.

What I'm trying to do here is get a rough quantification of the importance of excessive headspace being the issue.

[stonehog]

Actually - it's a full Odin BCG and bolt - here's the exact item I purchased:

https://www.joeboboutfitters.com/Odin-Wo...tg-bcg.htm

One other interesting bit about the pistol is I put a Strike Industries short pistol buffer tube with a 1.8oz buffer on it:
https://www.strikeindustries.com/si-ar-spre-slick.html

Would a light buffer potentially put extra strain on the case a bit early? It's a carbine gas system.

[LRRPF52]

Interestingly enough I too had doubts about "bolt Thrust" at one time, so I did the calculation running the cross sectional area of a case-body wall, and the tensile strength of brass. I had to admit, there is a meaningful degree of force required to stretch brass enough that it likely does decrease force on the bolt lugs' which, while no means the bulk of the force, it was a fairly nontrivial amount. I can't recall exactly, but vaguely I think it was in the 5-10% of force type of range - but don't quote me on that.

You’re talking about case obturation resisting bolt thrust due to it purchasing the chamber walls, effectively acting like a friction brake on the brass as it tries to flow against the bolt face.

Brass vs steel vs nickel-plated cases have different coefficient of friction in that formula, and case taper plays a role as well.

The more taper, the more force is exerted on the bolt face and lugs. 7.62x39 and .22-250 are some of the worse offenders in that aspect.

The lower chamber pressure 7.62x39 can have the same or more bolt thrust than 6.5 Grendel as result, though bore volume is much larger on 7.62x39 so it can drop pressure faster (.310” vs .264”).

The 7.62x39 conical shape focuses the force more than a straighter case like 6.5 Grendel. 7.62x39 is a much harder nut to crack in the AR-15 than people might imagine.

It causes more problems in the AK as well, compared with 5.45x39.

[Klem]

Stone,

What buffer you are using makes no difference to peak pressure if it was in battery - It is not a blowback system like in a submachine gun. By the time the bullet has passed the gas port and unlocking occurs, that's when the buffer make s a difference. By then the pressure will have dropped way below what it takes to blow a hole in a case. The buffer will influence things like harsh extraction from early unlocking but again, the pressures indicative of what you experienced rule the buffer out as the cause.

[LRRPF52]

I am aligning to your thinking. Basically I was evaluating the difference in how much a round, with an out of spec deep bolt and with excessive headspace, would add to the unsupported side wall area. 0.138" was your out of spec condition, and 0.133" was your in spec condition. This means at least 0.005" of case depth is being exposed to unsupported wall area, in addition to what is already exposed by the feed cone. I hope this makes sense now.

And then you could even add the headspace excess to it.

What I'm trying to do here is get a rough quantification of the importance of excessive headspace being the issue.

I’m tracking now. That makes perfect sense. You were basically establishing a window for tolerance deviation.

[DeNinny]

...
Would a light buffer potentially put extra strain on the case a bit early? It's a carbine gas system.

To me the buffer is not going to move until the bolt carrier moves. And the bolt carrier doesn't move until the gas hits it. And this doesn't happen until after the bullet is just past the gas port. This is well after it left the case. By that point, the pressure in the chamber is dissipating and past its peak.

So to me the buffer and buffer spring is not in play for this failure.

[DeNinny]

I’m tracking now. That makes perfect sense. You were basically establishing a window for tolerance deviation.

That's exactly it! And to further my point, I want to compare that added distance to the overall area that is already unsupported, which in theory should be the depth of the feed cone.

So, if anyone reading has the data on the depth of the feed cone for a standard M16 and for an Alexander Arms barrel, please chime in here!

[LRRPF52]

Lots of discussion here about Lapua brass and frankly I think it has gone sideways into a strawman argument thanks to the persistent arguing of one person (no offence). Just because the case has the most visible indication that something went wrong does not mean it was the cause of it.

We still don't know whether this was an in-battery or out-of-battery situation, and we also don't know whether headspace was in spec prior to firing. Headspace of the chamber/bolt combo, and headspace of the loaded case. We only know headspace is out of spec now. For a bolt to be in one piece after a catastrophic failure; the extractor is still OK, but out of spec now with a NO GO gauge introduces one possibility that the bolt was out of spec to begin with.

Another piece of info is the stretch of spent cases is .001", which is typical of bolt gun sizing. This introduces the possibility that the blown case might have had so little headspace that it interfered with the bolt trying to close. Yet closed enough to permit firing.

The load was compressed, and whether we think the amount is 'hot' or 'OK' is less important than it was compressed, with all that implies about changing expected pressures.

Regardless, I think we should move away from Lapua being a suspect brass manufacturer - for the simple reason they are not. Something other than brass manufacturer caused this.

Agreed. He successfully fired other loads through the firearm as well if I understand correctly, but also had Failure To Fire/Active Primer, difficult extraction of that cartridge. That’s either a firing pin protrusion issue, failure to go into battery, inactivated primer, or headspace on both too short or too long ends.

I will also point out again that 30 out of 40 published and pressure-tested max loads from Hodgdon’s are compressed.

Interestingly, only 4 of those 30 compressed loads reached 50,000psi, the highest being 50,100psi. Most of them are 49,900psi or less.

Several of the non-compressed loads exceed 50,000psi. Powder type was more factorial in whether 50,000psi+ pressures were reached.

[grayfox]

While Hodgdon doesn't say by how much the powder is compressed (only using a "C" for compressed load), Klem is there some indication you know of, how much that load might be compressed. I'm thinking that maybe 100.5%, vs. say 104%, the burn characteristics might not change much for the first, but a lot for the second...

[Klem]

While Hodgdon doesn't say by how much the powder is compressed (only using a "C" for compressed load), Klem is there some indication you know of, how much that load might be compressed. I'm thinking that maybe 100.5%, vs. say 104%, the burn characteristics might not change much for the first, but a lot for the second...

Gray,

I don't have a fundamental issue with compressed loads, it's just that you need to be careful. For years I used a drop tube, trying to cram as much 2208 (Varget) into a 223 case as possible. This, so that a heavy match bullet could reach 1,000yds with as much velocity as possible. Finish it off by tapping or holding against a tumbler, but you could still feel the crunch as the bullet went in. After 103% compressed you have to accept you are changing the fill/pressure relationship.

Look, I don't think the load being hot, let alone compressed is the main issue here. I'm just saying you cannot discount that it may have contributed to the case rupture, in a way a lighter load might not have. The OP had other tell-tale issues of failure-to-fire and stuck cases.

The OP reports a range of base-to-datum sized measurements: from 1.22 to 1.2301 (.01" range). This should not be an issue as long as that range is within the SAAMI range of max chamber 1.2301", and SAAMI's min ammunition spec 1.213" (so a .017" range). This assumes Odin has machined the chamber to SAAMI, and the bolt combines with the chamber to comply.

(Stone, I'd be careful comparing your Hornady headspace gauge readings to SAAMI's blueprint specs because unless your B350 insert is EXACTLY .350" ID then you are comparing apples with oranges).

My range of base-to-datum sized cases in this calibre is about 0.004" so the OP's 0.01" is a bit more generous but still potentially fine. Whether some of the cases are borderline too much headspace, made more possible by the chamber/bolt combination failing the NOGO (before firing), plus Odin's overly generous chamber mouth reaming not supporting the case enough, and maybe even a warm load supplying the pressure, is certainly possible.

[DeNinny]

Reference the photo of the cutaway 5.56 chamber in Page 3, Post #51. A 5.56 chamber first has a 120° cone, or ramp, cut into the breech leading into the chamber, and then the edges are radiused .0625. So, when visually examining the breech, you'll see the flat surface of the breech and then a relatively wide "circle" — that represents the angled cone and the radius — around the chamber mouth. The smaller that "circle" around the chamber mouth — representing a smaller angle and radius — the more supported the case web area.

Attached is a completely random and unrelated photo — I don't know what cartridges these barrels are chambering; I got the photo by Googling "AR15 feed ramps" — but the photo gives a rough side-by-side comparison of the visual differences of different "cone" sizes. The cone on the right, for example, is smaller than the left.

So I forgot that you had some data here regarding the feed cone specs. And it's very helpful in this analysis. But it's not perfect. So I have to make some assumptions which I will clarify along the way. Here goes...

To start, I've been wanting an approximate size of the feed cone depth because we are all mostly in agreement that this is the unsupported area of the case and the failure occurred at an unsupported area. Also remember that Bill Alexander himself stated that he had to reduce it for the grendel round. And per the note above..."the edges are radiused .0625"...I think this is a good starting approximation of the standard M16 feed cone depth which supposedly is the same for an Odinworks barrel. I'll note that it is not perfect, but I'm pretty sure the feed cone depth will be approximately close to this dimension.

Next, I want to compare this to the feed cone depth of an Alexander Arms barrel. But unfortunately, it was stated that this is proprietary information, so we will have to make an assumption here. And remember that Bill stated that this change was critical to the design because of the high pressure in the grendel round. So if it was this important, I think we could assume that he made at least a 30% improvement to reducing the size of the feed cone depth. So taking 70% of 0.0625", I am getting 0.7 x 0.0625" = 0.04375".

So now, let's take the difference between a standard M16 style feed cone depth and this assumed Alexander Arms feed cone. 0.0625" - 0.04375" = 0.01875". This is approximately how much more unsupported the OP's brass was when he switched to using his 12" Odinworks barrel which was reported to have the standard M16 feed cone design. Note also I'm assuming that his other barrels, which had no issues with this load, had feed cones more aligned to Alexander Arms style. That's why the load never failed on those barrels and it did on his Odinworks barrel.

Now let's compare this number to the bolt face depth being out of spec, which is one of LRRPF52's failure models. In past posts, we determined that an out of spec bolt face may expose the brass to become unsupported by an additional 0.005". This comes from the difference from an in spec bolt face to an out of spec bolt face. I'm not going to share the math again since it is in my last post since this one.

Comparing the two numbers above I get 0.01875" vs 0.005". Clearly, the feed cone depth issue is more significant than the out of spec bolt face issue. It exposes almost 4x more brass to being unsupported as compared to a bolt face being too deep.

So to me, based on this, we cannot just assume this issue was just the bolt face issue. The feed cone depth for an M16 barrel design is still a significant issue. It simply exposes more brass to being unsupported per my comparison above.

At this point I want to mention that the excessive head space issue is in the wrong direction with respect to exposing the brass to unsupported wall area. Excessive head space is at the shoulder end of the case and allows the round to go more into the chamber rather than out of it. As such, I'm not accounting for this in my math and moreover I don't think it is a viable failure model...but I'm open to being convinced otherwise.

Now as to the brass itself. It definitely plays a role here and cannot be dismissed as well. With more unsupported brass exposed, weaker brass will tend to fail more often, and stronger brass will tend to fail less. To this point, the first question I have is...would the OP have had the same failure with Hornady or another brass besides Lapua? To this issue, it is still undetermined. But it is entirely possible. I know some folks are saying no way it was due to Lapua brass, but I'm sorry that's not being very objective without numerical evidence. We have 3 threads on this forum alone that are of Lapua grendel brass failing. We have Bill Alexander saying that he designed the grendel working with Lapua brass and he outright states that he had to change the feed cone design to limit the amount of unsupported brass with it. We also have visual proof that Lapua has less overall brass around the case head. (And the only counterarguments to this are non-numerical claims..."Lapua is the gold standard of brass"..."I've used Lapua brass for years without problems"...etc.)

And finally, we have the OP using Lapua brass with an Odin barrel which has the M16 feed cone design that Bill Alexander intentionally redesigned for the grendel round.

So with all that said, all my earlier failure models are on the table and I'm in alignment with lazyengineer that it's a tolerance stacking issue...the barrel feed cone was too long, the brass was too weak*, there was too much powder, and (possibly) the bolt face was too deep. All these combined led to the failure.

*And note that by brass being too weak, I'm open to the fact that there may have been a defect in it and/or other factors besides that it was simply Lapua brass.

Lastly, I'm fine if anyone reading wants to refute this. That's part of having an open forum discussion! And I know we are all working on limited information and have to make assumptions to make progress. Also note that I'm simply trying to understand this failure for safety reasons. Mine and others. Also note that I'm trying to be as objective as possible using limited available numerical data. If you want to criticize me, please provide alternative failure models and provide some data and theory based on the laws of science that can actually be evaluated.

[lazyengineer]

So I forgot that you had some data here regarding the feed cone specs. And it's very helpful in this analysis. But it's not perfect. So I have to make some assumptions which I will clarify along the way. Here goes...

To start, I've been wanting an approximate size of the feed cone depth because we are all mostly in agreement that this is the unsupported area of the case and the failure occurred at an unsupported area. Also remember that Bill Alexander himself stated that he had to reduce it for the grendel round. And per the note above..."the edges are radiused .0625"...I think this is a good starting approximation of the standard M16 feed cone depth which supposedly is the same for an Odinworks barrel. I'll note that it is not perfect, but I'm pretty sure the feed cone depth will be approximately close to this dimension.

Next, I want to compare this to the feed cone depth of an Alexander Arms barrel. But unfortunately, it was stated that this is proprietary information, so we will have to make an assumption here. And remember that Bill stated that this change was critical to the design because of the high pressure in the grendel round. So if it was this important, I think we could assume that he made at least a 30% improvement to reducing the size of the feed cone depth. So taking 70% of 0.0625", I am getting 0.7 x 0.0625" = 0.04375".

So now, let's take the difference between a standard M16 style feed cone depth and this assumed Alexander Arms feed cone. 0.0625" - 0.04375" = 0.01875". This is approximately how much more unsupported the OP's brass was when he switched to using his 12" Odinworks barrel which was reported to have the standard M16 feed cone design. Note also I'm assuming that his other barrels, which had no issues with this load, had feed cones more aligned to Alexander Arms style. That's why the load never failed on those barrels and it did on his Odinworks barrel.

Now let's compare this number to the bolt face depth being out of spec, which is one of LRRPF52's failure models. In past posts, we determined that an out of spec bolt face may expose the brass to become unsupported by an additional 0.005". This comes from the difference from an in spec bolt face to an out of spec bolt face. I'm not going to share the math again since it is in my last post since this one.

Comparing the two numbers above I get 0.01875" vs 0.005". Clearly, the feed cone depth issue is more significant than the out of spec bolt face issue. It exposes almost 4x more brass to being unsupported as compared to a bolt face being too deep.

So to me, based on this, we cannot just assume this issue was just the bolt face issue. The feed cone depth for an M16 barrel design is still a significant issue. It simply exposes more brass to being unsupported per my comparison above.

At this point I want to mention that the excessive head space issue is in the wrong direction with respect to exposing the brass to unsupported wall area. Excessive head space is at the shoulder end of the case and allows the round to go more into the chamber rather than out of it. As such, I'm not accounting for this in my math and moreover I don't think it is a viable failure model...but I'm open to being convinced otherwise.

Now as to the brass itself. It definitely plays a role here and cannot be dismissed as well. With more unsupported brass exposed, weaker brass will tend to fail more often, and stronger brass will tend to fail less. To this point, the first question I have is...would the OP have had the same failure with Hornady or another brass besides Lapua? To this issue, it is still undetermined. But it is entirely possible. I know some folks are saying no way it was due to Lapua brass, but I'm sorry that's not being very objective without numerical evidence. We have 3 threads on this forum alone that are of Lapua grendel brass failing. We have Bill Alexander saying that he designed the grendel working with Lapua brass and he outright states that he had to change the feed cone design to limit the amount of unsupported brass with it. We also have visual proof that Lapua has less overall brass around the case head. (And the only counterarguments to this are non-numerical claims..."Lapua is the gold standard of brass"..."I've used Lapua brass for years without problems"...etc.)

And finally, we have the OP using Lapua brass with an Odin barrel which has the M16 feed cone design that Bill Alexander intentionally redesigned for the grendel round.

So with all that said, all my earlier failure models are on the table and I'm in alignment with lazyengineer that it's a tolerance stacking issue...the barrel feed cone was too long, the brass was too weak*, there was too much powder, and (possibly) the bolt face was too deep. All these combined led to the failure.

*And note that by brass being too weak, I'm open to the fact that there may have been a defect in it and/or other factors besides that it was simply Lapua brass.

Lastly, I'm fine if anyone reading wants to refute this. That's part of having an open forum discussion! And I know we are all working on limited information and have to make assumptions to make progress. Also note that I'm simply trying to understand this failure for safety reasons. Mine and others. Also note that I'm trying to be as objective as possible using limited available numerical data. If you want to criticize me, please provide alternative failure models and provide some data and theory based on the laws of science that can actually be evaluated.

That's pretty much where I'm at. the Feed cone thing I can't really measure or know how a typical user is supposed to be able to tell if he's in spec or not. Of the 3 Kabooms, all of them were with Lapua brass, and 1 of them with Odin Works BBLs, one with a PSA BBL, one was with a bolt action rifle. I don't have an Odin, but I do have an Alexander ( our benchmark), a Faxon (A known-good; yes?), and a PSA. So I took some shots, that I guess don't really say a lot, since one can't really tell the actually cut angle, or if there's a face-taper on top of that; nor the headspace. But, for reference all the same...

Alexander Arms - known good and developer of 6.5 Grendel

[lazyengineer]

Faxon Arms - (new in wrapper); assumed good, as part of the original Monster TEAM BBL supplier; which are considered known-good.

[lazyengineer]

PSA - (heavily used and obviously in need of a wee bit of cleaning); This brand is reportedly involved with one of the 3 Lapua involved Kabooms in the last year. Obviously this one hasn't. FWIW, its diet predominantly of Wolf steel and Hornady, FC, and 7.62x39 brass.







TBH, I can't really discern a lot of difference or draw a lot of conclusions from the 3 sets of photos, but maybe someone else will see something of note.

[DeNinny]

I'm glad we are on the same page, lazyengineer! And great pictures.

Unfortunately since the pics aren't set up the same and aren't always in focus, it's difficult to assess their relative differences. I will say that they all look deeper than 0.005" and I feel confident that my 0.0625" depth assumption isn't out of the ballpark.

Also, if we could get the radial thickness measurement of the cone from any one of those pictures, and knowing the cone's angle, we could get a second estimate of the cone depth. Using good old trigonometry I think we could calculate the depth.

[grayfox]

I don’t think we know the angle in deg, but might be able to measure the diameters at the breakpoints, IOW the inner, chamber diameter should be close to constant but the outer diameter of the innermost radiusing… smaller diameter I would infer that it’s cone is smaller. Larger dia would infer a deeper radius, if the cut angles are all equal. It’s 4 am, hope my brain is thinking straight.

edit to add, I don’t think it is correct to say lapua brass was weaker as we don’t know the alloys nor the yield strengths for any of the brasses, but it can be said that the brass is thinner so could be more susceptible to a hot gas failure. And the gas pressures for a carbine system vs rifle length system, a 12” will have most likely a clgs, pressures at unlock in clgs are higher than in a longer gas system.

[Klem]

I don’t think it is correct to say lapua brass was weaker as we don’t know the alloys nor the yield strengths for any of the brasses, but it can be said that the brass is thinner so could be more susceptible to a hot gas failure.

Only if the brass is weaker, and as you say, we don't know it is.

Once again I stress caution about doubting Lapua. We still don't know exactly what happened, let alone Lapua was responsible for it.

[DeNinny]

I don’t think we know the angle in deg, but might be able to measure the diameters at the breakpoints, IOW the inner, chamber diameter should be close to constant but the outer diameter of the innermost radiusing… smaller diameter I would infer that it’s cone is smaller. Larger dia would infer a deeper radius, if the cut angles are all equal. It’s 4 am, hope my brain is thinking straight.

edit to add, I don’t think it is correct to say lapua brass was weaker as we don’t know the alloys nor the yield strengths for any of the brasses, but it can be said that the brass is thinner so could be more susceptible to a hot gas failure. And the gas pressures for a carbine system vs rifle length system, a 12” will have most likely a clgs, pressures at unlock in clgs are higher than in a longer gas system.

Even if we just knew the radial thickness, as a first approximation we could assume the angle is 45 deg which would make the depth the same value due to the trigonometry. A right angle triangle with the other sides at 45 deg angles will have the same lengths. This would give us a ballpark size.

By weaker brass I meant it to be all encompassing until we have more data. This is exactly why I said just weaker 'brass' as opposed to saying weaker 'Lapua brass'. I wrote it that way for a reason...need more data. As I stated it could simply be a few defective cases. OP stated other brass had the belt phenomena.

And I completely agree it could have been heat! I posted this theory earlier. OP said it was round #41 of 50. OP said the belt phenomena happened to at least one other case before the failure. We all know a barrel gets hotter during a shooting session. To me heat is most definitely on the table! And it is also encompassed in my general statement of 'weaker brass'. Added heat will weaken the metal initially by making it softer, assuming it's not hot enough to change the molecular structure of the metal.

And I absolutely would LOVE to see all the metallurgical and materials science data between Lapua and other brass! I mentioned this earlier too. I want to see the Young's modulus, shear modulus, tensile strength, hardness, and various other factors on all manufacturer's brass! They would give me a much clearer idea as to which brass, all other things equal, is stronger and weaker. Until then we can only speculate, which is why weak brass is still on the table.

And please note that *if* we do find that they all have the same materials science factors, then for sure the less brass at the head for Lapua makes it inherently weaker at that point.

[grayfox]

This post makes me go back to see if I could find an article on brass I read a while ago...
Found it in Accurate Shooter...
X-Ray Spectrometry of Cartridge Brass



Re-printing the article here, but you can read it yourself at the link above. Does not have strength values but the Alloy series # (C24000 etc) should more than likely give at least the approximate values thereof, from standard industry sources. Probably won't indicate any heats, or other end-treatments a given mfr may do to their brass. Among the disclaimers is you can't infer any brass is better or worse than the other, from the data or from this article. Note that in Lapua's case, there are 2 entries. I believe "blue box" is the prevalent one today.

[Start]
Are there significant metallurgical differences in the alloys used in various brands of cartridge brass? The answer is yes, and we have proof. Using a state-of-the-art X-Ray Fluorescence Spectrometer, some tech-savy Wisconsin shooters recently analyzed the alloys in seven different types of cartridge brass.

The test results revealed significant differences in the percentages of copper and zinc in the different brands. Copper content ranged from a low of 72% by mass (Winchester, S&B) to a high of 80% by mass (Remington). Zinc, which adds hardness to the alloy, ranged from a low of 20% by mass (Federal) to a high of 36% (‘brown box’ Lapua). Interestingly, the tests, as reported by Forum Member Fred Bohl, revealed that the alloy in the new ‘blue box’ 6mmBR Lapua brass is different than the alloy in Lapua’s older ‘brown box’ 6mmBR brass. Specifically, the ‘blue box’ 6mmBR brass has more copper and less tin (by mass). Here’s a summary of the X-Ray Fluorescence spectrometry tests:

(the pic made this post too big, have to try pasting in on next post.)

This testing was done at major science laboratory, using high-grade X-Ray Spectrometry Analyzing equipment. Fred reports that: “The data was run by one of the club members with the permission of the test lab supervisor who is also a club member and shooter. The data in original output reports was far more detailed about trace elements at lower orders of magnitude primarily from surface contaminants (some were rerun after establishing a repeatable cleaning procedure)”. The testing process is discussed in this Shooters’ Forum thread.

We do NOT have the metallurgical expertise to infer that any particular alloy shown above is “better” than another. The alloy “blend” is merely one of many variables that can have an impact on the performance and quality of the finished product. Annealing times/methods differ and some cartridge brass is extruded while other cartridge brass is made with the traditional drawing process. Readers should not presume, on reading the above chart, that they can identify the “best shooting” brass simply based on the constituent metals in the various alloys.
General Observations about Cartridge Brass Alloys
With the cartridge brass X-Ray Spectrometry results in hand, Fred Bohl hoped to find out what “real world” conclusions (if any) we could draw from the raw data. Fred sent the test results to some knowledgeable metallurgists, soliciting their comments. Fred explains: “When I first posted this information [in the Shooters’ Forum], I had hoped to elicit replies from expert metallurgists and to initiate a useful discussion. From [their replies] I distilled the following ‘consensus’ comments”:

1. The range of Copper/Zinc ratios suitable for use in cartridge making by typical processes is 85/15 to 65/35 (% by weight or mass).
   
2. The range of Copper/Zinc ratios suitable for use in cartridges intended for reloading is 80/20 to 70/30. Above 80% copper, the resulting case would tend to be too soft and difficult to attain the distribution of hardness desired (harder at the base and softer at the neck). Below 70% copper the resulting case would tend to be too hard, would work harden too quickly and require frequent annealing. [Editor: That said, the ‘brown box’ 6mmBR Lapua brass, with 62% copper/36% zinc content, enjoys an unrivaled reputation for both accuracy and its ability to perform well after a dozen or more reloading cycles. We know 30BR shooters who have shot the same old-style Lapua brass (6mmBR parent case) more than 50 times. So maybe the “expert” view needs re-thinking.]
   
3. As the percentage of zinc increases, the tensile strength, yield strength and hardness tend to increase. However, above 35% zinc, while tensile strength will continue to tend to increase, both yield strength and hardness will tend to begin to decrease.
   
4. The trace additives of iron and/or silicon are used to control the processing characteristics of the alloy. Trace additions of chromium will improve corrosion resistance and give a shinier surface (both largely cosmetic).
   
5. Selection of the alloy and additives is a trade off among: end use desired properties; processing time and yield; and cost of materials. For example, the classic 70/30 cartridge brass was considered an optimum combination of corrosion resistance and hardness for single use by the military with good process yield at acceptable material cost.
   
6. All of my responding experts were surprised by the brown box Lapua alloy except for the oldest. He remembered using an almost identical alloy late in WWII when copper was in very short supply for military small arms ammunition.
   

[grayfox]

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