ORIGINAL: speyrjb
This is obviously a head space problem.
The solution I would have is to buy brand new brass and only trim them to length. Find a box of throw away bullets and load them very light. Shoot them at a burm or something and then going forward only neck size this brass about 2/3 down. You will have "fire formed" brass for this specific gun. Only use this brass in that gun.
This should fix your problem.
If you still have head space problems, then take it to a gun smith and have them see if the problem is fixable. Would probably require a new barrell, or posibly a custom bolt to fix.
This is obvoisly
NOT a headspace problem, if the shoulder on your brass is moving forward,
but the bolt face to belt recess shoulder is giving the correct support to permit your firing pin to indent the primer corrrectly in this push-feed action!! It WOULD be a headspace problem, however, IF IT WERE A RIMLESS, NON-BELTED CASE and the shoulders were moving forward on firing!
Despite this technical point, the solution recommended by speyrjb is correct, JUST SIZE ABOUT 2/3 of the neck after initial firing!!
HOWEVER!! IF you wanted to really go to the trouble to find out
if what we are talkng about is correct, you could
pretend that your rifle is chambered for a rimless NON-BELTED bottlenecjk case, and, using a .338" or .358" expander ball, neck one new case up, then neck it back down to .30,
LEAVING A FALSE SHOULDER ON THE NECK, the location of which you determine by screwing the sizing die out about 1/4", then screwing it back in about a quarter turn at a time while continuing to try to chamber the case each time you move this false shoulder back a tiny amount, until you get to the point where you can just close the bolt on this case with a little force. You now have a "correctly heaspaced" piece of brass, which is headspacing on the false shoulder, not the belt!!
Next, after you know where the false shoulder must be, put one like it on all the other cases and size them to where they can be chambered but just barely! Now you have some cases with "proper headspace", and they will not stretch when they are fired for the first time, nor ever aftward, so long as you NEVER LET THE FULL-LENGTH DIE PUSH THE SHOULDER BACK AGAIN AFTER YOU HAVE FIRED THEM ALL!
This is how I make cases for the .338 Gibbs, for example, using .35 Whelen cases, necking them down to .338 until they will just chamber then fireforming the full-size, .454" diameter, 35-degree Gibbs shoulder onto them!
(In my experience, the liklihood of having a sizing die that is out-of-spec is a lot higher than buying an American made rifle that left the factory with excessive headspace, or a modern-made American rifle in a caliber like .300 Win. Mag. that developed excessive headspace after it was purchased!!)
Let us know how this turns out???