LRRPF52 Wrote: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.

Quote: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.

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

LRRPF52 Wrote:I’m tracking now. That makes perfect sense. You were basically establishing a window for tolerance deviation.

Klem Wrote: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.

grayfox Wrote: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...

BluntForceTrauma Wrote: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.

DeNinny Wrote: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.

grayfox Wrote: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.

grayfox Wrote: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.

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.