Ka-boooom and stumped

[Mike-Sid]

My first guess would be out of battery detonation, but you rule that out. My only other guess would be excessive...real excessive head spacing.

How does this sound?

[DonT]

If it is not too much trouble can you measure the distance from the base to the sharp angle of the cartridge where the neck meets the body of shell and then measure this on one of your loaded unfired rounds (one that has not been fire formed) and then one that has been fired at least once in the rifle.

It would be interesting to see if the distance changed in any of the cartridges.

Also I pulled the bolt out of my AR to look at the chamber area and mine appears to have a slight angle to guide the nose of the bullet into the chamber with a very small radius where this breaks into the chamber. As near as I can tell everything in front of the extractor groove on the round is support by chamber, it is hard to see as I don't want to pull my barrel. maybe if someone has a barrel that is not installed they can try it with a factory round. In the photo of your chamber, and maybe it is just the photo, your chamber appears to have quite a substantial radius. If this is the case is it leaving the portion of the cartridge right ahead of the extractor groove unsupported??? Just curious.

Thanks,
DonT

[alpine44]

Yes, the "entrance" radius on my barrel is quite generous. But that turned out to be a good thing as it allowed the case to blow through the extractor cut-out and vent the pressure. Otherwise, the bolt would have failed. Keep in mind that the pressure was WAY over the design limit as indicated by the massive flow of brass.

I have added one picture to show how the barrel geometry will allow the bulged case head to fit between the barrel and a closed bolt.



Around Christmas, I will post more photos with actual case dimensions compared to a unfired and a "normally" fired case from the same gun.

Thanks to all for the input and suggestions.

[mstarling]

WOW ... I find that much unsupported case to be unsatisfying to say the least.

How did you measure the charge put into the cases? Lee dippers?

If so, have you measured the actual weights of say 10 to 20 charges using the same dipper? Would be very interested in knowing what kinds of charge weights you were actually getting (extremes and average).

As I said before I have used weighted 15.5 gr charges of W296 behind 147 gr FMJBTs ... and they were stiff in my 16.5" bbl. I backed down some for fireforming with the gas port closed.

[amafrank]

A couple possiblities not mentioned.....
the tracer bullet is much much longer than the standard ball bullet of the same weight. That creates much more drag and would indeed raise the pressure. If you had a little extra powder it probably wouldn't create much trouble with a normal bullet but a little extra and the long tracer might just do the trick. Another possibility is that H110 is too fast for that bullet and the combo of fast and hot could be just enough to create the blowout. I agree with some of the others that the chamfer/radius is more than generous and is in fact excessive. There is a lot of unsupported case that could contribute to a blowout. I do agree with your position that this is not the problem in the current case in question. There is a lot of excess with the primer and case head in that condition. The ejector formed a nice dimple as well with the case head flowing into the bolt face. I've seen some of this when using powders that are too fast even though the charge weight should be ok. 2400 seems to go that way with just a .1gr increase past what was working ok. I haven't blown any out to the extent you've shown but I have had some definite pressure signs with flowed case heads and primers. Another thing I've noted is that what works good in the bolt gun may blow the semi. I keep brass fired in the SIG separate from anything fired in a bolt gun due to differences in the case support which change how much stretching there is. Probably no simple answer to your case so use caution when using tracers or other large light bullets.


Frank

[Expatriot]

One thing nobody has mentioned is the possibilty of the neck being too long (case OAL too long) which would cause the bullet to be pinched into the neck when chambered. That would def cause an overpressure condition.

What are the chances that round had a magnum primer rather than a normal primer? That could also cause an overpressure condition.

Excess headspace does not cause excess pressure but it can cause the case to rupture. This issue was clearly caused by a massive overpressure.

[amafrank]

Expat is correct that a long case neck could create a pressure problem though it appears that the overpressure is pretty extreme and excess powder is still the most likely culprit.

One other suggestion is that you reduce your picture sizes. When you post the huge pics the screen can't contain it all and the posting has to be scrolled back and forth on a lot of our computers. If you crop them down to size that will help and not reduce the clarity.

Frank

[travelingman]

No more pics?

[miker557]

When I saw your pictures and read your description, I was reminded of something that happened to me. Many years ago I bought a Plainfield Enforcer M-1 Carbine Pistol. I shot a lot of ammo through it, but would frequently have problems with cases bulging at the base. I even had a few rupture, as yours did, at the base (and even blow out the bottom of the magazine, dumping spring and rounds all over my feet). I originally thought I had gotten some bad ammo, but after using different brands and having the same thing happen, I realized it was the gun. I invested in tooling to check my suspicions, and when I got it, they were confirmed.

It had excessive headspace.

I put a new barrel on the pistol, confirmed proper headspace, and it shot fine after that.

So, here's the $64,000 question - have you checked the headspace of your chamber? I have read that chamber dimensions vary slightly between JDJ's Whisper and the 30-221 that others make. Fortunately, I believe headspace gauges should be easy to find - I think that 221 Fireball gauges work for the 300 as well.

Give it a try. If nothing else, you'll eliminate that as a possible source of your problems.


Miker

[A62Rambler]

I think we have several factors working together. One we have a bullet with more bearing surface. Two we have clear indications of excessive pressure. Since the AR gas system works by gas pushing the carrier back and pushing the bolt forward and since the carrier was found to be partially back, I think we can conclude that there was still too high pressure when the gas began the process.

I think you should check the following: Were you using brass that had previously been fired with other loadings in this chamber? When you resized these cases did you check the case length?

The reason I ask is that if these cases had stretched or were too long plus the bullet with a greater bearing surface with even a slightly high powder charge you could have this result. I would take a close look at the bolt lugs to see if they show any damage. Pressure of powders when graphed is not a single line growing at a constant rate. At some point they jump at a much sharper incline. With the three factors above (long case, longer bullet, slightly higher charge) you could have hit the point in the pressure curve where you jump greatly in pressure for a small increase.

I hope that makes sense.