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

[LRRPF52]

You know what I can tell from this picture?



Extraction contributed to the failure.

What does that tell me next? Cyclic rate is too fast. The bolt is trying to rip the case out of the chamber too soon while the case is still obturated to the wall.

If I missed it already, I’m sorry but was this suppressed? Brass looks too clean for suppressed, unless a flow-through can was used.

Either way, CLGS needs to be managed carefully.

The way I deal with it in the 12” Grendels I have built is with Bootleg Adjustable Gas carriers.

I choke the gas down to the suppressed setting, and they cycle wonderfully.

I also used increased weight action springs. On one, I used a Franklin Armory carbine spring + power spring.

On the other, I used the Maxim Defense PDW shot RET with tiny buffer, but it has a really short, stiff spring.

Both have been shot pretty much exclusively suppressed with high efficiency (high back pressure) TBAC cans.

I have shot a lot of 120gr Federal, 123gr American Gunner, and 123gr ELD-M through mine.

My buddy has shot a lot of factory 123gr ELD-M and 8208 XBR under 107gr SMK hand loads.

No malfunctions so far through either suppressed.

* I wonder what your gas port diameter is, what carrier weight you have, buffer weight, and action spring type/length.

The discussion about Lapua brass has really distracted us from being able to focus on the real problems here.

I don’t think you have a headspace problem, but still want to see where it is.

Your other fired cases are the same lengths as mine that have come out that way for 13 years.

If your fired headspace is within spec, we’re looking at early extraction/too fast cyclic rate.

That is even more likely using a medium-slow-burning powder like AA2520 with CLGS under a 123gr, which is a ball powder that will hit the port with higher port pressure.

This makes perfect sense as to why your extractor didn’t blow out now.

Another question: Do you have an O-ring around your extractor spring?

My guess is that you probably do.

[DeNinny]

Extraction contributed to the failure.

I have been thinking that but with extraction happening well after the bullet has left the mouth, I see it as the extractor did the damage well after the actual kaboom.

To your point, though, the casing might have ballooned into the extractor itself and the sharp edge of it actually cut into the brass and that is where the initial blowout occurred. Then later as the case continued to tear, it tore at the feed cone score "line".

Can you elaborate on your own failure model here and see if our thoughts line up?

[LRRPF52]

I have been thinking that but with extraction happening well after the bullet has left the mouth, I see it as the extractor did the damage well after the actual kaboom.

To your point, though, the casing might have ballooned into the extractor itself and the sharp edge of it actually cut into the brass and that is where the initial blowout occurred. Then later as the case continued to tear, it tore at the feed cone score "line".

Can you elaborate on your own failure model here and see if our thoughts line up?

Extraction can’t physically transpire until the projectile has passed the gas port, otherwise there is no way for gas to flow back into the Stoner expansion system and cause carrier inertia.

No carrier inertia, no bolt movement other than thrust.

Extraction caused this case to fail. I know with absolute certainty the extractor ripped the case open because of the extractor shelf mark seen on the case head, clocked perfectly with the case shear.

This means the front of the case was happily stuck to the chamber wall still, under significant residual pressure.

The slower-burning AA2520 propellant hit the port, and the port is probably cut too large.

This allowed too much gas to flow into the expansion chamber too early in the cycle, causing the bolt to rotate and unlock, while the extractor held tightly to the case rim for dear life (O-ring installed, otherwise it would likely have just slipped off and caused a FTExtract).

The power of the excess gas was so significant, it drove carrier inertia so quickly that it had sufficient force to rip the rim off partially.

We wasted about 8 pages talking about brass strength and case webs basically, when the answer was right in front of us, had we been able to see the case head like that.

[Zeneffect]

Page 4, post 71. You guys and your hate for quickload and immediate dismissiveness is a bit unfounded if you know how to use the tools appropriately. We all know where "in the curve" we should see the gas port. Regardless of what any number software says, it's relative position is the clue since peak pressure is already known. We are keyboard theorists and can never determine with absolute certainty the case of failure, but we can use ALL of the tools available to get to the same conclusion faster with lack of additional evidence. In cases for load development, I have recently experienced both published load, quickload, and verified from others load involving TAC. If I fif not have my magnetospeed attached, I would have happily shot everything instead of stopping to say wtf. This was for .223 remington, where software and published loads should be most refined.

I dont trust the software with absolute certainty as I know these cases exist, but to say it's useless is simply not true. It has its uses (especially quicktarget) when applied appropriately.

Hell I don't trust anything basically until I've tested it for myself. I've been told way too many things that are simply not true I've adopted the stance that nothing is true until it is proven to be so.

[DeNinny]

[ATTACH=CONFIG]19151[/ATTACH]

Ok so I was able to enlarge the photo of the first failed case until it was 37.5" across my monitor. then, per my blue arrows, I measured the unsupported head area (or feed cone depth) at approximately 2.25". Scaling this down to the actual casing size I get an estimate of 0.0915" for the unsupported head area. Here is my math:

(2.25"/37.5") x 1.5245" = 0.0915"

Based on the failure score line location, this is my estimate of both the feed cone depth and the unsupported case wall depth. This is fairly close, bigger actually, than what I estimated from Bill Alexander's statements about the M16 style feed cone design.

So now taking 30% of this, I get .3 x 0.0915 = 0.027". <- This is the estimated additional feed cone depth for a standard M16 feed cone from an Alexander Arms feed cone, assuming that at least 30% improvement was achieved when Bill Alexander redesigned it.

This still puts this failure model as more significant than the bolt face depth out of spec model which was estimated at 0.005". Instead of 4x difference from my earlier estimation, it is now 5x different.

As such, the feed cone depth issue is still a viable failure model. This shouldn't be a surprise since Bill Alexander acknowledged it was an issue to begin with.

[LRRPF52]

It’s very likely that the feeding cone and bolt face depth is totally fine.

We need a protocol of sorts when reporting catastrophic malfunctions, which are actually rarely reported here.

It’s a good thing to build a data set on your firearms for reference anyway.

[LRRPF52]

[ATTACH=CONFIG]19151[/ATTACH]

Ok so I was able to enlarge the photo of the first failed case until it was 37.5" across my monitor. then, per my blue arrows, I measured the unsupported head area (or feed cone depth) at approximately 2.25". Scaling this down to the actual casing size I get an estimate of 0.0915" for the unsupported head area. Here is my math:

(2.25"/37.5") x 1.5245" = 0.0915"

Based on the failure score line location, this is my estimate of both the feed cone depth and the unsupported case wall depth. This is fairly close, bigger actually, than what I estimated from Bill Alexander's statements about the M16 style feed cone design.

So now taking 30% of this, I get .3 x 0.0915 = 0.027". <- This is the estimated additional feed cone depth for a standard M16 feed cone from an Alexander Arms feed cone, assuming that at least 30% improvement was achieved when Bill Alexander redesigned it.

This still puts this failure model as more significant than the bolt face depth out of spec model which was estimated at 0.005". Instead of 4x difference from my earlier estimation, it is now 5x different.

As such, the feed cone depth issue is still a viable failure model. This shouldn't be a surprise since Bill Alexander acknowledged it was an issue to begin with.

That’s a piece of brass that was ripped out of the chamber while it still had residual pressure on it, (but still didn’t exceed case length).

Has nothing to do with the feed cone or bolt face, and everything to do with early extraction from cyclic rate being ahead of the window.

We were wasting time looking in the wrong places.

[LRRPF52]

Page 4, post 71. You guys and your hate for quickload and immediate dismissiveness is a bit unfounded if you know how to use the tools appropriately. We all know where "in the curve" we should see the gas port. Regardless of what any number software says, it's relative position is the clue since peak pressure is already known. We are keyboard theorists and can never determine with absolute certainty the case of failure, but we can use ALL of the tools available to get to the same conclusion faster with lack of additional evidence. In cases for load development, I have recently experienced both published load, quickload, and verified from others load involving TAC. If I fif not have my magnetospeed attached, I would have happily shot everything instead of stopping to say wtf. This was for .223 remington, where software and published loads should be most refined.

I dont trust the software with absolute certainty as I know these cases exist, but to say it's useless is simply not true. It has its uses (especially quicktarget) when applied appropriately.

I don’t have any emotions about QuickLoad. I just recognize that the developer(s) never had 6.5 Grendel pressure trace data with a test breech, and that their generated peak and subsequent down-barrel pressure levels are starting with peak pressured that are 9,000-13,000psi different from actual calibrated test breeches with this cartridge.

So not only are the chamber pressure models invalid and not useful, but the predicted port pressure values are way off too.

If you took available test breech data and programmed it into QL for 6.5 Grendel, as well as down-barrel piezoelectric measurements, and generated a nice data set that way, then QL/6.5 Grendel would be very useful for this cartridge. Whatever modeling they have in the program is basically useless though and invalid, especially as a diagnostic tool in this situation.

[DeNinny]

Extraction can’t physically transpire until the projectile has passed the gas port, otherwise there is no way for gas to flow back into the Stoner expansion system and cause carrier inertia.

No carrier inertia, no bolt movement other than thrust.

Extraction caused this case to fail. I know with absolute certainty the extractor ripped the case open because of the extractor shelf mark seen on the case head, clocked perfectly with the case shear.

This means the front of the case was happily stuck to the chamber wall still, under significant residual pressure.

The slower-burning AA2520 propellant hit the port, and the port is probably cut too large.

This allowed too much gas to flow into the expansion chamber too early in the cycle, causing the bolt to rotate and unlock, while the extractor held tightly to the case rim for dear life (O-ring installed, otherwise it would likely have just slipped off and caused a FTExtract).

The power of the excess gas was so significant, it drove carrier inertia so quickly that it had sufficient force to rip the rim off partially.

We wasted about 8 pages talking about brass strength and case webs basically, when the answer was right in front of us, had we been able to see the case head like that.

We are aligned on the extraction happening after the bullet passes the gas port. That's what I mean by well after the bullet has left the mouth.

Also I understand your model, but the other failure model is also supported by this evidence that I posted earlier...

The case head expanded into the extractor and the blowout occurred while still in battery and it started at either the sharp edge of the extractor or it started at the score line where the unsupported wall starts.

Also remember...by the time the extractor is actually extracting, all the pressure is already released from the casing. The brass is no longer subject to the severe pressures as it was when the bullet was just leaving the mouth.

Also...note that there is zero extractor damage to the other two failed casings. To me these two earlier failures (but not blowouts) are an indication of a pressure issue at the head. The pressure is pushing the limits of the brass and there were two signs of this before the blowout.

I'm sorry but there are holes in the extraction failure model, and there are other models that still support the failure as shown in the picture.

[DeNinny]

That’s a piece of brass that was ripped out of the chamber while it still had residual pressure on it, (but still didn’t exceed case length).

Has nothing to do with the feed cone or bolt face, and everything to do with early extraction from cyclic rate being ahead of the window.

We were wasting time looking in the wrong places.

I respectfully disagree. The other failure model is still viable and fully supported by the picture.

And I'm fine if we agree to disagree. It's been a great thread and I've personally learned a lot for my own reloading purposes myself.

[LRRPF52]

The case is a partial case head separation from incorrect/excess cyclic rate.

Notice how the case shear is clocked perfectly with the extractor location.

Lapua brass camouflaged the issue because of its strength.

Had these been a weaker brass alloy, I suspect the failure would have happened much earlier.

[Zeneffect]

I don’t have any emotions about QuickLoad. I just recognize that the developer(s) never had 6.5 Grendel pressure trace data with a test breech, and that their generated peak and subsequent down-barrel pressure levels are starting with peak pressured that are 9,000-13,000psi different from actual calibrated test breeches with this cartridge.

So not only are the chamber pressure models invalid and not useful, but the predicted port pressure values are way off too.

If you took available test breech data and programmed it into QL for 6.5 Grendel, as well as down-barrel piezoelectric measurements, and generated a nice data set that way, then QL/6.5 Grendel would be very useful for this cartridge. Whatever modeling they have in the program is basically useless though and invalid, especially as a diagnostic tool in this situation.

I would agree that peak pressure modeling is iffy at best for both 6.5 and .223, but since we already know what the expected peak pressure is we aren't calculating it, we are simply following a predicted curve.

You can think of it this way. With your experience, take a piece of graph paper and approximate the pressure curve and designate the peak as 52kpsi by simply drawing it. Now place the positions of gas ports relative to pressure curve. You will get the exact same visual data, pressure is too high. In essence you are quickload in modeling. Accuracy here does not matter, and data is still valid enough for diagnostics.

Im not saying that it's any more accurate than just eyeballing it... but since the delta is so big, it's pretty easy to do and accurate enough. Microscopic levels of resolution and precision are not needed in a case where simple demonstration of a past event with no consequence to gather theoretical data to support a postulation. Been a good read so far though.

[DeNinny]

Notice how the case shear is clocked perfectly with the extractor location.

I pointed this out earlier and again the brass could have easily expanded/ballooned/flowed into the extractor. The blowout model while still in battery could have caused the damage just like we see.

[DeNinny]

[ATTACH=CONFIG]19152[/ATTACH]

Ok so I enlarged the view of the other failure and I want to point out a few more things that support the blowout (while still in battery) model.

Based on this picture, the blowout looks to have started where my red arrow is indicating. The brass is literally blown out and there is a flap section of it further away from the tip of the red arrow. This section was the weak point which again is right at the unsupported wall score line.

And the tearing of the brass continued along the score line from the unsupported wall. It ended approximately where my blue arrow is pointing.

Again, the blowout model is still at play here. IMHO.

Edit: Remember too, that the extractor part of the bolt has more room for the brass to expand than all the other areas where the lugs are. The extractor tip is literally smaller than a bolt lug, so the brass is going to expand there first due to the path of least resistance for it.

[Zeneffect]

Not criticizing, but is that picture super small for anyone else?

I think early unlock still. After staring at it, to me the flap occurred after blowout as that's where the initial separation started, and saw the most pressure as the seam continued radially chewing the brass with powder, hot gas, and fire. If the extractor was in the center of the fissure, then that makes even more sense to me.

I see the bolt opening under too much pressure, extractor is now hooked in good, and the brass is still stuck to chamber walls. Slop in bcg tail has caused lateral shift due to the case being stuck and is now exerting even more pressure on extractor area beginning to thin it faster than the rest of the case as it's trying to pull at an angle now. (Imagine if it were bubblegum because it basically is at this point)

This resulted in a weaker area relative to the rest of the case resulting in tear beginning at that very spot and resulting in a new path for fire and brimstone to spew while the bolt is still ripping the case further.

Take a spent good case and blue it. Stick it in chamber to see where exactly on the case its unsupported. I bet it's .5-1mm behind your blowout.

We CAN measure some stuffs right? 11 pages and all I see is a spent case length lol

[LRRPF52]

The case head didn’t expand into the extractor. Look at this image:



The extractor was pulling with such force on the case, that it stretched the side wall, causing the side wall to literally rip apart with a fine, thin line.

I couldn’t see this from the initial pictures, but the more recent one shows it clearly where you can see the case head from the rear, as well as the tear.

Residual pressure folded the brass from that failure point backwards as the spent case was being extracted.

The is textbook partial case head separation, which is almost always a gas system issue in self-loaders.

Since these were first loads, virgin brass, incipient case web weakening isn’t an issue because the brass has not been hot and cold-worked repeatedly.

With a choked gas system or port work and heavier action spring, the problem should go away and brass will come out nice and clean, ready for many more loads.

You have to take the whole system into consideration, and we wasted 8 pages talking about things that I think were totally unrelated to the failure.

This was a major problem in the Colt 602 rifles in Vietnam before the M16A1 development corrected all those issues.

They had rough chambers, minimum or less-than-minimum spec chambers, lightweight Edgewater spring guides, certified with stick powder in the States, then sent to Vietnam and fed ball powder cartridges.

They were ripping case heads off, slipping extractors over rims and FTExtracting with next round feed into the stuck case, and partial case head separating.

His brass actually looks like the chamber is very clean.

Once the projectile passes the gas port and then leaves the muzzle, all the gas pressure has NOT subsided.

The gas volume is directed back through the tube into the expansion chamber with enough mass and expanding energy to shove that carrier rearward at extremely fast speed, faster than your eye can track.

There is enough residual bore pressure during plug dwell time before the projectile exits to maintain significant pressure on the brass.

The timing of these events is within tiny fractions of a second, so there is room for an excessively-fast cyclic rate to cause violent extraction and subsequent damage to the case head.

[DeNinny]

Not criticizing, but is that picture super small for anyone else?

I think early unlock still. After staring at it, to me the flap occurred after blowout as that's where the initial separation started, and saw the most pressure as the seam continued radially chewing the brass with powder, hot gas, and fire.

Sorry for resolution. You can look at OP's first post of the picture and you'll see the same thing there, without arrows.

I'm sorry but the flap could easily occur from the blowout itself, when the pressure is much higher than during extraction. Because of this, I think the blowout has a higher likelihood of creating that flap instead of the extractor pulling on it.

[DeNinny]

The case head didn’t expand into the extractor. Look at this image:



The extractor was pulling with such force on the case, that it stretched the side wall, causing the side wall to literally rip apart with a fine, thin line.

I couldn’t see this from the initial pictures, but the more recent one shows it clearly where you can see the case head from the rear, as well as the tear.

Residual pressure folded the brass from that failure point backwards as the spent case was being extracted.

The is textbook partial case head separation, which is almost always a gas system issue in self-loaders.

Since these were first loads, virgin brass, incipient case web weakening isn’t an issue because the brass has not been hot and cold-worked repeatedly.

With a choked gas system or port work and heavier action spring, the problem should go away and brass will come out nice and clean, ready for many more loads.

You have to take the whole system into consideration, and we wasted 8 pages talking about things that I think were totally unrelated to the failure.

This was a major problem in the Colt 602 rifles in Vietnam before the M16A1 development corrected all those issues.

They had rough chambers, minimum or less-than-minimum spec chambers, lightweight Edgewater spring guides, certified with stick powder in the States, then sent to Vietnam and fed ball powder cartridges.

They were ripping case heads off, slipping extractors over rims and FTExtracting with next round feed into the stuck case, and partial case head separating.

His brass actually looks like the chamber is very clean.

Once the projectile passes the gas port and then leaves the muzzle, all the gas pressure has NOT subsided.

The gas volume is directed back through the tube into the expansion chamber with enough mass and expanding energy to shove that carrier rearward at extremely fast speed, faster than your eye can track.

There is enough residual bore pressure during plug dwell time before the projectile exits to maintain significant pressure on the brass.

The timing of these events is within tiny fractions of a second, so there is room for an excessively-fast cyclic rate to cause violent extraction and subsequent damage to the case head.

I disagree. This picture fully supports what I've stated too. We will end up repeating ourselves now, so I'll leave it as we both have these models out there and others can take from it what they want.

[LRRPF52]

[ATTACH=CONFIG]19152[/ATTACH]

Ok so I enlarged the view of the other failure and I want to point out a few more things that support the blowout (while still in battery) model.

Based on this picture, the blowout looks to have started where my red arrow is indicating. The brass is literally blown out and there is a flap section of it further away from the tip of the red arrow. This section was the weak point which again is right at the unsupported wall score line.

And the tearing of the brass continued along the score line from the unsupported wall. It ended approximately where my blue arrow is pointing.

Again, the blowout model is still at play here. IMHO.

Edit: Remember too, that the extractor part of the bolt has more room for the brass to expand than all the other areas where the lugs are. The extractor tip is literally smaller than a bolt lug, so the brass is going to expand there first due to the path of least resistance for it.

As the extractor pulled rearward with extreme force, the case wall thinned out and succumbed to the residual chamber pressure, even after primary extraction had occurred. The bolt is rotating almost immediately, so now shear forces are introduced as well, not just axial forces. Normally the rotational shearing force will contribute to a complete head separation and also break the extractor lip off of the extractor shaft.

This one all held together quite well. Normally if you have a case failure while totally in battery, the bolt gets welded or impinged to the extension by molten brass residue, and the gases will split the carrier in half.

Because primary extraction was in progress, it allowed the gases to vent out the receiver and down the magazine well, blowing out the magazine.

One of the most overlooked aspects of the AR-15 or any rotating bolt lock-up is the rotational force during an event like this, or even on guns that seem to run, but are excessively-gassed.

You can expect extractor failure with extractors getting beat up like that under tension from out-of-time actions.

It was a major problem for 14.5” M4A1s suppressed with the KAC can, as well as 10.5” guns-all of which are 5.56 NATO of course.

It led to the development of the LMT Enhanced Bolt Carrier Group, which elongated the cam helix in the carrier, and created a pre-vent gas relief in the gas path inside the carrier key/expansion chamber junction.

If you dropped that carrier into the OP’s gun, I doubt this would have ever happened.

[Zeneffect]

I'm saying the extractor pulling on it isn't the cause for the flap, that is from discharge of solid propellants, igniting propellants, and hot gasses. It starts as a crack then exahstrabates into a flap as that is where peak pressure started, and lowest pressure occurs directing outward gasses to largest diameter area (the initial small crack which was previously initiated by early unlocking)