.410 bore

The .410 is the one shotshell that is not named as a gauge. Every other British shot cartridge counts how many bore-diameter lead balls make a pound; this one is simply the bore in inches, because when Eley began loading it in the 1870s it was a small-game and garden-gun cartridge rather than a member of the gauge series. The continental name for it, 36 gauge, is a retrofit, and the arithmetic does not work: a genuine 36-gauge bore would measure about 0.506 in.

Everything about the round follows from a bore narrower than a .45 pistol barrel. A shot column of a given weight can only get taller, so a .410 load stacks its pellets several deep instead of spreading them across the base of the wad, and the column deforms more of them against the barrel wall on the way out. That is why the cartridge is loaded light — a quarter to eleven-sixteenths of an ounce, against an ounce and an eighth in 20 gauge — and why it is a close-range cartridge in practice as well as in reputation. The straight, rimmed case has the same head and rim on every shell in the family; only the length changes, and C.I.P. standardises six of them on one datasheet, from 50.7 mm to 76 mm.

The narrow case head is also what makes the round unusual outside shotguns. It is close enough in rim and base diameter to .45 Colt that a revolver chambered long enough for a .410 shell will accept both, which is the basis of a family of American revolvers and derringers sold as dual-calibre guns, and the 2½ in shell is the one those chambers are usually cut for.

Gaps on this page: the entry records the 63.5 mm shell throughout, because the head dimensions are common to the whole family and the case length is not — the 76 mm magnum shell is rated at 1050 bar rather than 830 on the same C.I.P. sheet, and that figure is noted rather than stored. Federal publishes the slug velocity without naming a test barrel, so the barrel length recorded with it is SAAMI’s 26-inch standard test barrel and is marked as an estimate; the velocity itself is the published one. No shot load is recorded at all, because a shot charge has no single projectile mass to enter and choosing one would be inventing a fact the cartridge does not have.