Mixing It Up

American riflemen freely use metric as well as Imperial terms. Remington had a whole metric series with the 5mm, 6mm, 6.5mm, 7mm and 8mm cartridges. Americans cheerfully mix the two systems, for example naming cartridges 6mm-.284/6.5-.300 Weatherby Magnum, or even mixing metric with years as in 6mm-’06 or 6.5mm-’06.

Rifle barrels are measured in two basic ways, bore diameter and groove diameter. Bore diameter is the interior diameter of the barrel as it comes from the drilling machine. A .30-’06 barrel, for example, has a bore diameter of 0.300", as do many other .30-calibers such as .30-30 Winchester, .300 Savage, .300 Weatherby. Typically the rifling grooves are cut slightly deeper as bore size increases. With .22 caliber bores, the rifling grooves are generally (not always — in the shooting world there are exceptions to about everything) cut to a depth of 0.003". Since grooves are cut on both sides of the bore the total depth is 0.006", so the groove diameter is 0.006" larger than bore diameter. Bullet diameter usually is the same as groove diameter.