Re: new build .300wm barrel legth?
I'm convinced one could go to 36 inches for any 30 caliber cartridge burning more than 70 grains of powder. I tested two .308 Win. barrel lengths, 22 inches and 32 inches, muzzle velocity went from 2800 fps to 3100 fps with the same M80 ball ammo I tested. In a 30" barrel, same load had a little over 3000 fps. All three had 4-grooves with about .3078 groove and .2990 bore diameters. The rounds had 41 grains of IMR4475 powder ('bout the same as IMR3031) under 147-gr. bullets.
I can imagine the big velocity increase from a .300 Win Mag barrel going from 24 to 30 inches with 70% more powder than a .308 Win. behind any bullet fired. There must be some reason why the 30 caliber magnums (including mine) used in 1000-yard prone matches had 28 to 30 inch barrels.