Here’s more wisdom from Roy F. Dunlap in Ordnance Went up Front. In this case, he’s noting that captured rifles often have mismatched bolts, and headspace may be an issue. But you might not have the gages you need for some oddball foreign round. How do you know it’s okay? He provides a quick and dirty method for checking bolt gun headspace. (We have separated his 1940s wall-o-text paragraphs into shorter ones for readability’s sake).
Not many gunsmiths have headspace gages in foreign calibers, but a practical test can be made with live ammunition. At least 10 clean loaded, unmutilated cartridges, preferably military cartridges of the same type and origin should be used.
The bolt must be stripped of all parts except the extractor collar, which should never be removed, and the magazine spring and follower removed. The chamber should be polished clean with cloth, and if any trace of rust is observed, clean it out with steel wool or even crocus cloth. Chamber, bolt head and lugs and the locking lug recesses in the receiver must be absolutely clean and dry, without any oil whatever. Any burrs on the bolt face around ejector cut or firing pin hole should be removed.
With the bolt and receiver ready, the cartridges are placed in the chamber one at a time and the bolt closed very gently, the knob of the handle held loosely between thumb and forefinger. Half of the cartridges should offer no resistance to the bolt’s complete closing, and two or three cause slight effort to seat fully. If the bolt flops shut on all the cartridges available, or will not close at all on any, do not attempt to use the rifle but send it to a responsible gunsmith for a thorough check with legitimate gages. consider whatever the charge is in the light of an insurance premium.
If the bolt is “felt” To close on at least one out of every five cartridges tried, headspace may be considered safe enough for test fire at least. Some idea of the headspace distance range maybe gained by using the cartridge on which bolt closes easiest and placing as many .001″ shims as possible between bolt face and cartridge base. If the tolerance is over .008″, ‘t’aint so good – get it checked with gages.
You can see, then, why it’s only really possible to do this test as described with bolt action firearms.
He goes on from there to describe causes and consequences of too little or too much headspace, and even provides schematic instructions for making one’s own “real” headspace gages — which he notes, is quite a tough proposition.