Could I neck-size-only my brass?
Don't ever neck size for this rifle unless you want to have an out of battery slamfire which knocks the back of the receiver off!
Projectile: Milsurp 147grn FMJBT
Primer: Federal standard large rifle
Case: HXP 68 (it has been loaded at least 4 times all necksized only after being fire formed)
COL: 3.292 (sample of lot that left)
Powder: 48 grains of IMR 4895
Dies: Lee neck sized
This is an M1 carbine receiver, easy to understand, functionally no different from a Garand. Notice that between the red lines nothing prevents the firing pin from making contact with the primer. You can take your Garand action out of the stock and verify the same condition.
This mechanism does not have a positive firing pin block, that firing pin is rebounding off a primer throughout the complete feed cycle, which is one reason you see a firing pin dent on the primer of a chambered round.
If you happen to have a real sensitive primer, and your case has been full length resized so that the lugs go into battery without any delay, your chances are good that you will have an in battery slamfire. But, if you neck size such that the bolt has to crush fit the cartridge to the chamber, that firing pin is rebounding off the primer, at the highest speed in the feed cycle, and your lugs are not engaged. If you have the bad luck to have a real sensitive primer than your chances are good to have an out of battery slamfire. With the exception of roller bolt mechanisms, all semi auto rifle mechanisms have the chance to slamfire, because they don't have firing pin blocks, and if the firing pin rebounds off the primer, they all will slamfire. Most of the later semi auto mechanisms do have safety features to prevent firing pin contact between firing pin and primer when the bolt is out of battery. There are lots of AR15 slamfires but that mechanism positively keeps the firing pin behind the bolt face, so AR15 slamfires are in battery. (might be a couple of out of battery but it was not due the firing pin) The perversity of the Garand mechanism, that is M1 carbines, M14's, M1a's, mini 14's, is that this mechanism allows contact between the firing pin and primer all the way to cam down. Therefore, full length resize and use the least sensitive primers you can buy. I recommend the CCI #34 primer or the Tula mil spec. Less sensitive is better than more sensitive.