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While the nominal bore diameter was .450 inches (11.4 mm), the groove diameter was actually closer to .458 inches (11.6 mm). As was standard practice with many early commercially produced U.S. cartridges, specially-constructed bullets were often "paper patched", or wrapped in a couple of layers of thin paper.
The means of measuring a rifled bore varies, and may refer to the diameter of the lands or the grooves of the rifling. [2] For example, the 257 Roberts and 250 Savage both use a .257 inch projectile; both 250 Savage and 257 Roberts rifle bores have a .250 inch land diameter and .257 inch groove diameter. [4]
The specified diameter of the throat may be somewhat greater than groove diameter, and may be enlarged by use if hot powder gas melts the interior barrel surface when the rifle is fired. [17] Freebore is a groove-diameter length of smoothbore barrel without lands forward of the throat.
The distance across the bore from groove to groove is greater than the distance from land to land. Projectiles fired from rifled barrels must be of the full groove-to-groove diameter to be effectively rotated by the rifling, but the caliber has sometimes been specified as the land-to-land diameter before rifling grooves were cut.
Bore diameter is given as 9.30 mm (0.366 in) and groove diameter is 9.55 mm (0.376 in). C.I.P. recommends a six groove barrel contour with each groove having an arc length of 2.92 mm (0.115 in) and a twist rate of one rotation in 305 mm (12.0 in).
While the bore diameter of 0.4880 in (12.40 mm) is consistent with other firearms which fire a 0.500 in (12.7 mm) diameter bullet, the groove diameter of 0.4983 in (12.66 mm) is an oddity as most firearms which fire a 0.500 in (12.7 mm) will have a groove diameter of equal to the diameter of the bullet. [7]
SAAMI recommends a barrel with a 6 groove contour and a twist rate of one revolution every 9 in (229 mm) with each groove having a width of .095 in (2.41 mm). The recommended bore diameter is .256 in (6.50 mm) and the groove diameter is .264 in (6.71 mm). SAAMI recommends that rifling commence at .221 in (5.61 mm) from the chamber mouth.
SAAMI and the CIP recommend a 6-groove barrel with a twist rate of one revolution in 10 in (250 mm) of barrel length with a bore Ø of .330 in (8.4 mm) and a groove diameter of Ø of .338 in (8.6 mm), with each groove having a width of .110 in (2.8 mm). The SAAMI-recommended average pressure for the cartridge is 64,000 psi (4,400 bar).