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The Truth About Overbore Capacity
In my formative years, I read a lot in P.O. Ackley’s books on wildcat cartridges, and as a result, I worried a lot about overbore capacity and barrel life instead of the war in Vietnam, nuclear conflict, and the price of gas, which had hit 75 cents a gallon. Overbore capacity, for those of you who are not familiar with the term, refers to a cartridge case that burns far more powder than it can efficiently use in exchange for a proportionately small increase in velocity.
This past weekend, I worked up a handload for my Ultra Light Arms .340 Weatherby, a rifle I’ve owned since 1993. The .340 is actually .338 bore diameter, but where a .338 Winchester takes about 70 grains of powder to give a 250-grain bullet 2,600 fps, my Weatherby uses 88 grains to push the same slug at 2,775.
Holy Barrel Burnout! Eighty-eight grains of powder is a load, and I thought there was no doubt that my lands and grooves were scorched to a cinder. So, filled with an overmastering sense of doom, I put the bore scope in the barrel and lo and behold, the rifling was as shiny and sharp as the day it left the Douglas factory.
The truth is that very few people shoot big magnum rifles unless they’re either crazy or have way too much money. Even with 14 years on it, that rifle has simply not had much use, and its barrel will last much longer than I will. Is the extra velocity worth it? Yes, marginally. Up to 300 yards it doesn’t make much difference, but beyond that, it can.