Cratered and blown primers with Black Hills ammo in an AI MKI?

[locolife]

Encountered something odd at the range today. While shooting an AE MKI .308 Win. with 180gr Black Hills Match ammo, the shooter experienced a series of misfires. Rounds appeared to have solid primer strikes but didn't fire. The rounds were chambered into another AI and they all fired. A closer look at the brass of the ammunition that he fired before the misfires revealed signs of overpressure. One primer had a hole at the firing pin and several other were severely cratered. I'm thinking the misfires were due to a primer fragment on the breech face but don't understand why the 180's were hot only in that rifle. The rifle fired 168gr Hornady TAP without a problem. Other AI rifles firing the same 180gr Black Hills round had no issues. Any ideas what it might be?

 

[Dynamic Response]

I would say the chambers headspace is a bit to tight and it's causing excessive pressure with those case. The other ammo shot fine because the cases headspace was less.

 

[locolife]

I was thinking the same thing so I measured several of each cartridge and found very little difference between them.

OAL from Ogive:
Black Hills: 2.1380
Hornady TAP: 2.1405

Shoulder length:
Black Hills: 1.5580
Hornady TAP: 1.5560

 

[Dynamic Response]

My dad's POF 308 had this problem. The headspace was so tight that Some ammo would shoot fine and others had signs of over pressure. He sent it back and it was the headspace causing the problem. It was technically in spec but the slight difference in cases made it to tight. The difference of .002 could put it over the edge.

 

[Dynamic Response]

Headspace from min to max is only .010. So if you are on the low end of that and you get a case that is .002 longer then most you are to tight. It's a fine line.

How does the bolt feel when chambering a round? Did you measure any fire formed cases?