Model 11 first rebarrel, headspace and primer issues
Hi all, first post here. I've built AR-15s and reloaded for about 4 years now, and a buddy and I have decided to branch out into bolt guns. Being left handed, I got my first Savage about 15 years ago and haven't looked back.
Since we already have ARs chambered in 6.8 SPC, it was natural to go that direction with our bolt gun project. Our barrels are chambered in 6.8 SPC II. The Spec II chamber has a different chamber design in the throat and freebore, all other case dimensions are the same. We built the first one this weekend, and something doesn't seem right.
Donor rifle was a new M11 Trophy Hunter in .243. We installed the new 6.8 barrel, new recoil lug, and barrel nut. PTG 6.8 bolt head was used. Headspace was set with PTG 6.8 SPC go and no-go gauges. Bolt would just close on the go gauge, not on the no-go.
We took the rifle out to test fire and sight in. We used Hornady and SSA factory ammo. Brass showed significant case stretching (~.004"), case bulging, flattened and cratered primers. We tried the other factory ammo, same result. We fired one of our starting charge reloads with similiar "overpressure" indications.
Brought the rifle home and tore it down. Set the headspace with a sized piece of brass, using the "scotch tape" method to set the headspace. At this point, we tried the go gauge. The bolt will not come close to closing on the go gauge. After much discussion and thought, we decided to shoot the rifle, as it chambered factory ammo fine. The sized brass we used to set headspace is the same dimension as we use for our AR-15 reloads, and have no issues with it. Reassembled and back to the range.
After firing the factory ammo again, brass looks normal. Normal looking brass, no more stretch issues, however the primers are still cratering. Flattened primers have gone away, we did not see them again until we approached the max charge on our load development. Barrel shoots very accurately.
Long story short, I'm not real comfortable with the bolt not closing on the go gauge. I know it chambers factory ammo from both the manufacturers we normally shoot, but this doesn't feel right. There was approximately 3/4 of a turn difference in the barrel between the headspace from the go gauge and the headspace using the sized brass. Granted, everything feels right, based on the way the brass looks. Should we be concerned about this?
As far as the cratered primers, they crater no matter factory or reload, starting charge or max charge. They start to flatten as we get to the max charge, which is normal. Since we changed the bolt head, could firing pin protrusion be the cause here? I can't think of any other reason, other than an oversized firing pin hole, that would cause cratering. I'm applying AR logic here, as that is a not uncommon problem with an AR, but this is a bolt gun. Same idea, different mechanics.
Any insight would be greatly appreciated. At this point, the rifle shoots great, brass looks great, but we are still cratering primers.
Thank you in advance.
