Yeah it closes nice and easy, so I'm now concerned about how much space is between the bolt and the back of that chambered round. I guess if it fires, it's not an awful amount, right? What's the worst that could happen?
It should not close on a field gauge. Sorry I can't help you. Did you buy it new? If it's new surplus it should have been checked. Pasadena gun center has a smith and I bet they have a field gauge and will check it for a nominal fee. Be sure to call ahead. The smith is not there full time.
I own a gun with excess headspace that I've never fixed.
Accuracy is degraded.
I get some misfires when the firing pin energy is wasted pushing the cartridge further into the chamber instead of setting off the primer.
Sometimes cases stick in the front of the chamber and the primers back out. (Obviously, this isn't a high-pressure round.) With factory ammo when the cases don't stick towards the front, they display flattened primers that look like the ragged edge of high pressure, almost more than I will tolerate when working up loads. It's weird to fire two factory rounds in succession where one has a flat primer, looking like a high pressure load, and the other has a primer backed 1/3 of the way out and still possessing a rounded profile at the rear. It's very weird.
Overall, I can never escape the feeling that I'm on the verge of something really bad. I don't shoot that gun any more. Maybe someday I'll get around to getting it fixed.
My conclusion is that firearms with excess headspace should not be fired even though I've done it with no particularly bad results. Besides, if you think there's a problem, then there's a problem. Find the gauges and put your mind at ease.
I doubt you'll have any problems. Maybe a split case. If that bolt head came with the mosin I'm sure its fine. The mosin fires a rimmed cartridge (54R). Its pretty forgiving on head spacing. I'd fire it and check the case to see if it splits.
There are many ways to lengthen a cartridge... duct tape... clay... use your imagination. I'd think you'll know if it's close without any real trouble. The 303 British generally is long in the chamber and the round simply grows to fill the chamber without issue since both the 7.62x54R and the 303 British headspace on the rim.
I've actually seen a 300 Win Mag after it was shot in a 300 Weatherby Mag chamber. No damage or signs of any trouble, but it sure is "funny" looking.