Safety and Headspace
On used bolt-action firearms, especially, the headspace should be checked with headspace gauges prior to shooting to ensure it is correct, and to prevent over-stressing chambers and cartridge brass. Some bolt-action rifles, such as the Lee-Enfield, have a series of different length bolts available to extend the service life of the rifle, for taking up any wear of the bolt and chamber occurring from long years of service. In the case of the No. 4 Lee-Enfield bolt, the bolt heads themselves are replaceable separate from the bolt and are marked 0, 1, 2, or 3, with each bolt head in sequence being nominally 0.003" longer than the bolt head numbered one less, for easily taking up any action stretching that may have occurred. It is possible to replace such a bolt head without tools by disassembling the bolt from the action, unscrewing the bolt head, and replacing the bolt head with the next higher number bolt head, for restoring a safe headspace.
The interrelated mechanics of safe trigger function, correct headspace, and equal bearing of the locking lugs requires that the bolt and action assembly are factory "fitted". Usually shown by the rifle serial number, applied to both bolt and action, indicating they are a matched pair. Accidental or deliberate swapping of bolts between similar rifles is not unusual, but is potentially dangerous. Any rifle with mismatched action/bolt serial numbers should be considered to be unsafe to fire until checked and so marked by a competent gunsmith or armourer.
Furthermore there are many subtle issues involving the provenance of a rifle and its ammunition. Many calibres have dual civilian/military uses but are not completely identical - e.g. the .308 Winchester/7.62mm NATO and .223 Remington/5.56mm NATO have very slight differences in chamber sizes. Military ammunition often has thicker brass, and harder primers. Over major wars there were literally millions of surplus rifles converted to civilian uses (sporterized), many may be unsafe with modern ammunition - caution is required with any ex-military bolt action.
Read more about this topic: Bolt Action
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