Common sense "guess" would be that it's deformed when cutting the slot/clearance for the extractor, given that it's basically turned to sheet metal...
I'm gonna get over my OCD on keeping clearances on the minimal side here- ordered a .710 counterbore.
I do believe that you can have too much clearance in this application, as keeping it minimal would seem to be critical to containment in the event of a casehead failure. If I were going to cut a mile of clearance, no reason to have the counterbore at all, just keep a flat breech.
I ain't saying there's anything wrong with a flat breech either, but there is that liability "thing" as we know...
When a cartridge goes rouge and decides to emulate an IED the function of the rifle becomes all things associated with venting that uncontrolled pressure. The quicker that happens, the less likely the shooter is to eat it in the face. I discovered this personally about 6 or so years ago.
The original Bighorn actions made in CO had a bolt head retaining pin that was a bit anemic. The non-rotating breech plug behind the bolt head basically acts like a piston in an engine when a case gives up the goods. The pin sheared and the bolt shot out of the back of the receiver, snapping the fire control in half when it contacted the cheek piece. I ate the bolt head just below the right eye. An orbital fracture and torn retina along with a half dozen or so blown light bulbs throughout the shop from the shrapnel.
If the "piston" had not been there, I'd of likely just gotten peppered with some powder/funk. I really wish manufacturers would rethink this pathway.
-A friend of mine owns a Top Alcohol Funny Car that I get to make stuff for sometimes. A few years back he got a block made from a bad casting. Run #4 resulted in the block splitting in half. That dropped the crank. When it came back up the cam timing was now out of phase and it resulted in a "sneeze" going out of the intake system. The blow-off valve worked as designed, preventing the supercharger from lifting off the motor. What it also did was pressurize the entire cabin area. The brand new, $30,000.00 carbon fiber body blew off the car. Smile, wait for flash, instant convertible....all in about a 10th of a second. That was roughly a $140,000.00 fuck up.
The car now has a line of sight pathway from the blast plate to outside of the car. -For the exact same reasons, I'm describing here. Experience is never cheap.
Anyways. Clearance is a good thing as it checks a couple of boxes. 1, is the circumstance I just talked about. 2, is the need to make room for crap. Carbon, debris, mud, etc. . . The more room there is for this stuff to be squished someplace other than between the bolt nose and the breech ring, the better.
The ultimate expression of this is when the counterbore is completely removed from the barrel.
Savage, ARC, etc. . . the list is long. Nobody has any issue with the bolt "flopping around" in that environment. There's no reason to be "boxed into" the thought process that it has to happen here either.
Open the shit up. Your face will thank you someday.
C.
