Originally Posted by backwoods83
Bart I know your sole purpose in life is to prove others wrong, but last I checked a standard 30 cal barrel was .300 on the lands and .308" on the grooves, since the lands start the engraving how does this occur between .307-.309", do tell oh wise one!
First off, check out the chamber and bullet specs for the .300 Wby Mag at:
and you'll see the SAAMI specs for the chamber throat diameter is .3084" to .3094". Its leade angle is 1 deg. 2 min. (that'll vary as much as 20% across the chamber reamers) and tapers down to the groove diameter of .3080" to .3090". Finally down to the bore diameter of .3005" to .3015".
Bullet diameter specs range from .3053" to .3083" so I can imagine the low resistance and peak pressure if a .3053" diameter bullets shot through a barrel with a .3015" bore diameter. The opposite's the case with maximum diameter bullets in a minimum diameter bore and groove barrel. And differences in ogive radius also have an effect.
Other variables is the bore cross sectional area where the grooves can vary in width as well as numbers. The bore cross sectional area has a SAMMI spec minimum of .0736" square inches but it can be a lot bigger and that'll mean lower peak pressures for a given load.
Barrels made to SAAMI specs will have these area's dimensions somewhere in the range listed above. But all 30 caliber barrels and .300 Wby Mag chamber reamers are not made to SAAMI specs.
Engraving starts where the leade first contacts the bullet. Get your calculator out then figure out where the leade starts touching the jacket of several bullets with different diameters and ogive dimensions. Or coat a bunch of bullets with machinists layout dye, gently push them into the bore, carefully remove them then measure their diameter where the barrel marks them. Start with Sierra's HPMK bullets some of which are .3084" diameter. You may already be able to figure out where a .3090" diameter inch 30 caliber bullet will first start being engraved in a .3094" diameter throat and a 1 degree leade angle. Then you'll see where first contact between bullet and barrel starts.
It doesn't take but a few ten-thousandths dimensional or a few minutes of angle in the physical properties of chambers and bullets to change the diameter a few thousandths inch on the bullet where the barrel first touches it.