11-23-2021, 06:02 PM
Oh Wow! Glad you are OK.
FWIW, I run 27.8 gr ARComp under 120 SMK. And they run fine in my 20". I will say though, you have to be careful with Grendel with OAL. Many loads that are magazine length, are still too long and will engage in the lands. Which isn't good, as that can cause a pressure spike and accuracy issues, so not sure if you tested that. But I have to run my OAL closer to 2.22", or something like that, to clear, in my own chamber. I also don't use Lapua brass. That brass likely has a smaller capacity, which will mean a bigger P spike. Those two factors combined could result in a spike, but to be honest, I still wouldn't expect a complete Kaboom like that. The head is completely sheared off.
Cause is not obvious right now. Could be a combination of partial barrel obstruction being a brand new barrel, P spike from wedging into the lands, P spike from case-volume, and P-spike from that round being on the higher end of the tolerances of your load practices. From your description, it sounds like the pressure released definately happened within the first few inches at peak (well, by definition, at peak), and so the bullet didn't have full energy of acceleration for the dozen inches, and so it impacted very low.
So I guess some basic questions:
Show us what the head and case bodies of the other rounds look like
Show us what the remaining loaded rounds look like
Pull apart 5 rounds and carefully measure the powder weight, and examine for any inconsistency or contamination. Does the powder look like AR Comp?
Go ahead and examine the bullets - are they all really 120's, or did a heavy slip in there? (not likely, but they're out in your hands anyway)
Test your seating depth in that chamber (Hornady tool, or just make a bullet with a sharpe and see if it gets scratches on the ink by engaging the lands)
Was the barrel cleaned prior to firing?
Was the new barrel inspected prior to firing - even if cleaned, to assure a patch or other residue wasn't in there?
Was this a slam-fire, or did you pull the trigger?
Check your remaining loaded roads in that upper = does the upper readily close and rotated on them? Do they readily come out, when you cycle the charging handle?
Tell me about your brass processing - could there have been left over debris of some sort in them? (thus reducing case capacity) I've had that happen once, but usually obvious when the powder all comes spilling out and won't fit.
PSA customer service is actually quite good, so it's possible they will take it back for inspection or replacement, though it sounds like nothing was damaged. I find it interesting the casing ruptured and failed, before the bolt lugs did. I don't know what to make of that. If it were an OOB, that would mean the 1/4" or so of clearance betten an open bolt and rotated bolt was somehow bridged by something longer than your firing pin. That would be remarkable, and I have my doubts. But who knows, being brand new, was there debris that got launched into the firing pin upon the snap on the firing pin?
Don't know - often these are obvious, and I'm keen to learn what caused this one. (FWIW, in most Kabooms today, the first qustion is "do you own a .300 blackout", and much of the time, that's where it is - but obviously not the case here!)
FWIW, I run 27.8 gr ARComp under 120 SMK. And they run fine in my 20". I will say though, you have to be careful with Grendel with OAL. Many loads that are magazine length, are still too long and will engage in the lands. Which isn't good, as that can cause a pressure spike and accuracy issues, so not sure if you tested that. But I have to run my OAL closer to 2.22", or something like that, to clear, in my own chamber. I also don't use Lapua brass. That brass likely has a smaller capacity, which will mean a bigger P spike. Those two factors combined could result in a spike, but to be honest, I still wouldn't expect a complete Kaboom like that. The head is completely sheared off.
Cause is not obvious right now. Could be a combination of partial barrel obstruction being a brand new barrel, P spike from wedging into the lands, P spike from case-volume, and P-spike from that round being on the higher end of the tolerances of your load practices. From your description, it sounds like the pressure released definately happened within the first few inches at peak (well, by definition, at peak), and so the bullet didn't have full energy of acceleration for the dozen inches, and so it impacted very low.
So I guess some basic questions:
Show us what the head and case bodies of the other rounds look like
Show us what the remaining loaded rounds look like
Pull apart 5 rounds and carefully measure the powder weight, and examine for any inconsistency or contamination. Does the powder look like AR Comp?
Go ahead and examine the bullets - are they all really 120's, or did a heavy slip in there? (not likely, but they're out in your hands anyway)
Test your seating depth in that chamber (Hornady tool, or just make a bullet with a sharpe and see if it gets scratches on the ink by engaging the lands)
Was the barrel cleaned prior to firing?
Was the new barrel inspected prior to firing - even if cleaned, to assure a patch or other residue wasn't in there?
Was this a slam-fire, or did you pull the trigger?
Check your remaining loaded roads in that upper = does the upper readily close and rotated on them? Do they readily come out, when you cycle the charging handle?
Tell me about your brass processing - could there have been left over debris of some sort in them? (thus reducing case capacity) I've had that happen once, but usually obvious when the powder all comes spilling out and won't fit.
PSA customer service is actually quite good, so it's possible they will take it back for inspection or replacement, though it sounds like nothing was damaged. I find it interesting the casing ruptured and failed, before the bolt lugs did. I don't know what to make of that. If it were an OOB, that would mean the 1/4" or so of clearance betten an open bolt and rotated bolt was somehow bridged by something longer than your firing pin. That would be remarkable, and I have my doubts. But who knows, being brand new, was there debris that got launched into the firing pin upon the snap on the firing pin?
Don't know - often these are obvious, and I'm keen to learn what caused this one. (FWIW, in most Kabooms today, the first qustion is "do you own a .300 blackout", and much of the time, that's where it is - but obviously not the case here!)