In the 1950s NATO sought to standardise on a common cartridge for its armies, replacing the .30-06, .303, 7.92 and various other calibres then in service with the alliance. After long and at times acrimonious trials which saw such promising calibres and rifles such as the .280″ British EM2 discarded, NATO settled upon the 7.62x51mm cartridge. For the civilian market, Winchester developed the similar .308″ round and the two have often been regarded as identical ever since.

In practice, the rounds are not identical and though it is generally safe to use military 7.62x51mm ammunition in rifles chambered for the .308″ Winchester cartridge, the reverse is not always the case. Problems can arise from a number of small but important differences between the cartridges and the rifles designed to use them:

• Military rifles generally have looser chambers than civilian rifles. This allows them to chamber rounds made by different arsenals which could be at different ends of the permitted manufacturing tolerance band. The same chamber will also accommodate a round that may have expanded a fraction due to prolonged storage in desert climates as easily as a round that could have been stored under Arctic conditions. Furthermore, a slightly looser chamber is less likely to seize and jam in the adverse conditions of mud or dust. In military service, reliability is given a higher priority than absolute accuracy.

• Military cartridges tend to be made from thicker brass than civilian cartridges, again primarily for  reasons of reliability and robustness in storage and transport. This reduces the quantity of powder that can be contained within the cartridge, and also influences the pressure characteristics of the burning propellant.

• Military cartridges tend to be loaded to a lower maximum pressure than civilian cartridges, again to allow them to cope with wide variations in climatic and storage conditions without generating excess pressure.

• The headspacing on the 7.62x51mm differs slightly from that of .308″: the 7.62 NATO “go’ gauge is 1.635″ and the field reject gauge is 1.6455″ whereas the .308 Winchester is 1.630″ and the reject is 1.638” inches.

In practical terms, all 7.62x51mm NATO rounds should chamber and fire in a .308″ rifle, and ought to do so without causing problems. Power and accuracy will be less than with a .308″ round but there is unlikely to be any safety issue.

Most .308″ rounds will fit in a 7.62x51mm chamber but will generate higher pressures than would a 7.62x51mm round in the same chamber. Depending upon the metallurgical quality and service history of the rifle, there could be issues arising from overpressure. If, on loading a .308″ round in a rifle chambered in 7.62x51mm NATO, there is any indication of it being a poor fit, or a fired cartridge displays indications of over-pressure, for example flattened primers, it would be wise not to continue using .308″ cartridges in that gun.