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.
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".
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.
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