The extreme quickness of a round firing and its tight grip against the chamber wall puts far less force against the face of your bolt than most shooters might think.
At most it'll be 1/10th or so as much as the psi level of the peak pressure.
Extreme low pressure rounds are forced forward more due to the detonating of the primer than firing pin inertia.
In my rifles with in-line ejectors pushing on the case heads on chambered .308 cases, that pushes them all the way forward until they stop against the chamber shoulder. Then stay there while firing pin drives them hard into the chamber shoulder setting the shoulder back a few thousandths. This is easily seen and measured with an empty sized case chambered, a rod put down the muzzle, then a caliper measuring its back and forth play as it's pushed back on the case moving it against the bolt face then releasing it and watching it get pushed forward by the ejector spring's force on the ejector pushing on the case head. The amount of movement's equal to head clearance; the difference between chamber headspace and case headspace (distance from case head to case shoulder datum)
This was proved in one of my tests by priming empty cases with dud primers (putting oil or water on them) then chambering and pulling the trigger so the firing pin smacked them with full force in several .308 rifles with in-line and exteranal ejectors. Every instance had the case shoulder set back as measured by an RCBS Precision Mic for case headspace before and after firing those primed cases. Changing firing pin strength from 2 to 26 pounds showed even the lightest one drove the case full forward in the chamber. Nothing held the case back. There's all sorts of ways to easily measure this and see exactly what happens. I don't think it's hard to figure it out without measuring anyway.
Tests with live primed cases showed the same amount of shoulder setback. None of the primers backed out of their pockets. Evidence to me that live ammo does the same thing.
I once loaded 44 rounds in a match but only 22 fired; 20 had no powder, no bullets left the case mouth nor primers backed out from the duds. Subsequent tests with a few more rounds of that lot that weighed 44 grains less than others (proof there was no powder in them) showed the case shoulders were set back a couple thousandths by firing pin impact. That was issued ammo, not stuff I handloaded.
Fired cases can actually get shorter, but in my experience, that's mainly because of chamber diameter being wide enough (compared to your case) to draw the brass deeper inside the chamber.
A friend in 1970 once had shot 20 V's in a row in a 1000 yard match with his .300 Win Mag but had no more ammo to continue shooting for the record. Someone gave him a handful of ammo to continue shooting; his first shot with it had the bullet hit the dirt in front of the target. He ejected that round noting it was a .30-.338 Win Mag. The fired case looked much like the .300 Winnie fired in a .300 Wby chamber in your picture (which isn't clear enough for me to see any difference in case length). Years later, when I had a .300 Win. Mag., I fired one of my .30-.338 rounds in it to compare before and after case length. The fired case was several thousandths shorter after firing and had the same appearance as the ones you show.
These cases were held back by their belts stopping against the chamber headspace ridge so there was clearance between the case shoulder to the chamber shoulcer as the round fired. This doesn't happen with rimless bottleneck case whose headspace is shorter than chamber headspace.
Most case length shortening I've seen was when some 7.62 NATO ammo was fired in 30 caliber M1 rifles. The cases were held tight enough at their shoulder-body junction so they fired, but pretty much stayed in place and probably helped by the extractor's lip holding the rim while their shoulders and necks were blown out with the case looking like a regular .30-06 one trimmed back to about 1/16th inch in front of the shoulder-body junction. They were visibly shorter after firing compared to an unfired 7. 62 case. It can be seen in the image below:
The variation in case diameter vs chamber diameter doesn't need to be this extreme to create shorter fired cases. If resizing is done accurately, fired cases usually get longer.
Back to my earlier question; what do you think's in a rifle that holds a .308 Win. case back when it fires with its shoulder clear of the chamber shoulder to blow the case shoulder forward?
A couple dozen types of rimless bottleneck cartridge cases from 22 to 32 caliber I've reloaded all got shorter when fired. Moreso with new cases compared to full length sized ones.
Handgun cases are different, at least in my tests with 9mm, .38 Spcl, .357 Mag and .45 ACP; their primers usually end up backed out from primed cases being fired in them. Their firing pin impact force is much less than centerfire rifles have. Powderless ammo usually drives the bullet out of the case for some distance from pressure from just the primed case.
