Quarterbore's Forums - Ka-boooom and stumped

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Ka-boooom and stumped

After 20+ years of safe reloading I recently had a hair raising "event".

Loaded a 300 Whisper case formed from 5.56 Nato WCC brass, primed with CCI400, and charged with 14.4gr of H110 behind a 147gr FMJ mil surplus tracer seated to COL of 2.165". The AR15 DI action cycled OK but weakly, primer did not show any signs of pressure and the case was smoked about half way down. I concluded that more powder could not hurt and bumped the charge up to confirmed 15gr (the next cavity in the LEE powder measure) and got the results seen in the attached photos.
http://s557.photobucket.com/albums/s...m/300W_KB1.jpg
http://s557.photobucket.com/albums/s...m/300W_KB2.jpg
http://s557.photobucket.com/albums/s...m/300W_KB3.jpg
http://s557.photobucket.com/albums/s...m/300W_KB4.jpg
http://s557.photobucket.com/albums/s...m/300W_KB5.jpg

Fortunately, I followed my habit of firing new loads with the the gun held away from face and body. The external damage was limited to the magazine being blown apart. I was able to open the action by carefully prying the bolt carrier backwards with a large screw driver resting against the upper receiver mag cut-out. There is no visible damage to the barrel extension and/or bolt other than the bent extractor. I will perform magnetic particle and dye-penetration testing on the bolt to see if any cracks have formed. For now, the bolt has been "quarantined".

http://s557.photobucket.com/albums/s...m/300W_KB6.jpg
http://s557.photobucket.com/albums/s...m/300W_KB7.jpg
http://s557.photobucket.com/albums/s...m/300W_KB8.jpg

What happened here? Secondary Explosion?

I have worked up loads with 150gr FMJ to 17gr of H110 in the same cases and gun without ill effects. Granted, the bearing surface of the tracer bullet is larger, it will reduce the case volume due to its length, but that in itself should not cause one additional grain of powder more to have these devastating results.

We are not looking at a smoked or pierced primer here. This case has seen MAJOR overpressure befor it blew, judging by the severely expanded case head.

I have ruled out the following items to have caused the case blow out:

- Powder mix-up. I only keep one bottle on my bench at all times. (Have learned from dumping the powder hopper into the wrong bottle some 10 years ago).
- Double charge is impossible as the powder will overflow and the bullet will become impossible to seat.
- Obstructed barrel is ruled out as the previous bullet hit the target.
- Case neck diameter has been confirmed to be ok. Fired cases, including the blown one, will allow new bullet to be dropped in case.
- Bullet was confirmed to not sit against the lands. I wish I could get the mil bullets closer to the lands but in the barrel from Model 1 Sales you will run out of magazine space first.
- Soft case head. Unlikely, as this is once fired military brass that was not annealed after it left the factory. I have a hardness tester and could try to get a measurement on the mutilated case and on a comparable piece of another case from the same lot. However I am hesitant to destroy the "reloader's conversation starter". Also, if we look closely at the paper-thin and blown primer remnant we can point the finger at massive overpressure rather than weak case material.

I had an issue once where I dumped powder into a funnel and noticed something did not look right in the case. It had about 1.6 times the amount of powder that I had put in the funnel. The two cases before it had significantly less powder in them. The powder had gotten stuck in the funnel and finally dropped on that one case. I now check and see how many grains the loaded round is after it is loaded.

Alleycat,

Powder bridging is a plausible explanation at this point. I was using a progressive loader and did not visually check (shame on me) the powder level in each and every case for small rifle ammo. I do this now, of course. Weighing the completed cartridge is a great idea.

I have to check how much H110 would fit into the case without the relatively long bullet bottoming out. When I seat bullets, I do not just yank the lever down but feel what is going on.

On a bright note. This event has given me more confidence in the safety of the AR platform, having "grown up" around Mauser K98 actions.

Agree with Alleycat - have had it to happen on more than one occasion, but only with stick powder. It is amazing an ar could take that kind of pressure!

Lookin at the enlarged portion of the brass just above the case head looks to me like it could be an out of battery discharge. It seems like the brass has enlarged more than the chamber should have allowed. Just my .02.

H110 is so fine, I'd doubt suck funnel. It sure looks like you got 14.5g and then 15g more. I forget how much 14.5 grains fills the case, I been using VV110 and Lil'gun for a while, staying away from 296/H110.

I think 23grains is about all you could stuff into a case even if you were trying.

H110 and W296 are the only powders which I have seen with the warnings:

Quote:

For those loads listed where a starting load is not shown, start 10% below the suggested maximum load and then approach maximums carefully, watching for any sign of pressure (difficult extraction, cratered and flattened or blown primers, and unusual recoil). H110 and Winchester 296 loads should not be reduced more than 3%.

Reduce H110 and Winchester 296 loads 3% and work up from there. H110 and Winchester 296 if reduced too much will cause inconsistent ignition. In some cases it will lodge a bullet in the barrel, causing a hazardous situation (Barrel Obstruction). This may cause severe personal injury or death to users or bystanders. DO NOT REDUCE H110 LOADS BY MORE THAN 3%.

Those warnings are one of the reasons I am using LilGun most of the time, the other big reason is 110/296 are almost too fine, clog up my meter.

Hodgdon page with warnings

Looks to me very much like an out of battery discharge ... end of the case is expanded larger than the body of the case, and the bowout occurred in a place that would be unsupported if the case were not fully into the chamber.

In fact ... look at the rear section of the case ... you can see a line that is likely to be the end of the chamber and the chamfer on it!

Bad business this one!

You were very lucky to be sure.

Mike

I'm with Mike due to the case shape and damage to the extractor. It went off before it should have.....?

I don't know. It's not as ez to fire an AR out of battery as say an AK or SKS. The bolt has to rotate almost completely for the firing pin can protrude through the bolt. The primer is flat and has a deep indention. It is also in the case. I think the pressure was too high when the bolt unlocked. If it was not an AR I would be on board with the out of battery idea. A high primer may cause it to fire prematurely, but It should not have the pin dimple.

The bolt was 100% closed when the charge ignited. I let the BC slam home, pointed the gun away from me, and pulled the trigger. Also, I spent about 30 minutes to get the bolt open after the ka-boom. It needed a lot of lube to enable the bolt to turn together with the case. The bolt could not turn relative to the case because of the severe extrusion of brass into the ejector hole and into the extractor gap. The extrusion of brass into the ejector is another indication that the bolt was closed.

I agree with the posters that doubt real bridging on such a fine powder. But, I use the Lee universal powder die that has two brass bushings inside. These could cause trouble. Powder could get between them.

Putting 15gr on top of 14gr is impossible. Not enough space in case.