40 yo marlin 336. Havent shot much in the last 20 years. Not since my kids killed there first deer with it. Broke it out the safe for my grandson. Shooting factory corelokts and hornady ftx the primers are backing out. More pronounced on the hornady. Not sure if it’s always done this been so long. Had some old brass I could check but a crackhead helped himself to it a few years ago. Fyi new brass versus once fired is moving about .020. Yikes. Google says it’s not uncommon. Got some loads worked up on the fire formed brass so we’ll see. What ya’ll say.

It does happen more than you think.

Even with some factory loads in the 7x57.

Never ever had that problem with any 30/30
rifle or ammo.
Shot Remington, Winchester, Federal,
Hornady factory loads.
Marlin, Remington, Savage, Contender,
H&R/NEF rifles
My own hand loads forever plus 2 days
with every suitable powder and primer
from just barely a pop, to really shouldn't
be loading that with no problems.

Back in the 70's, I did have some factory
Federal loads that I experienced neck
splits with, but that's it.
I'll also add that I've loaded some of my
30/30 cases way more times than I've
kept track of, and they still work great,
aren't stretched, primer pockets still
tight

I have seen this many times with factory ammo out of low pressure cartridges. The 30-30 more than any other. You could really clean the chamber and maybe the cases would slide back and reseat the primers. Usually what happens is the firing pin hits the primer pushing the case to the front of the chamber, the primer fires, pressure pushes the primer back to the bolt face and then the case slides back to reseat the primer. I have seen many somewhat rough or dirty chambers cause the case to "cling" long enough that pressure drops past the point of sending the case back to seat the primer again.

True, a very common event, probably not dangerous. Headspace of any given rifle chambered for a rimmed cartridge, is the distance between the boltface and the rim recess of the barrel abutment. It was originally checked by closing the bolt on a soft lead wafer and measuring the result. But...my opinion only, any cartridge that headspaces on the rim, should not allow primers to back out. And if it does, the excessive headspace can only come from a stretched action frame or worn bolt lockup.

I've seen it up to .007" on an old h&r single shot that had a loose lockup. Shot great though.

If you handload you could try seating the bullets out closer to the lands. It would reduce the initial push from the firing pin strike. Trying a few different lengths would test that theory. Check the amount of primer protruding for each different length.

If your primers are backed out you have excess headspace, you do not have "fireformed" brass and the primers will again back out on your handloads.

A lot of 30-30 ammo is loaded well under SAAMI to cover for older abused rifles and generally weak actions. If you are shooting factory ammo only and not reloading the brass I wouldn't worry about it. Good explanatory article here.

https://gundigest.com/gear-ammo/ammunition/experimenting-with-bolt-thrust

It's not a problem at all. Really common in 30-30.

My Dad’s ‘94 (early post ‘64) backs out primers with pretty much every shot, factory or reloads. My ‘80 mfg 336 also does this with some factory loads, but not all. But my 2000 mfg 336 doesn’t back them out at all.