.318'' groove No.4 Enfield, what bullet to use?

[Bruce_in_Oz]

The .303 was DESIGNED as a black-powder cartridge, at about the same time the French were introducing smokeless powder.

The design of the Enfield rifling as shown in that drawing extract, is correct. Remember that in the late nineteenth century, boring and reaming a nice straight, mostly parallel hole was not much of an engineering challenge. Cutting FIVE, equally-spaced grooves of EQUAL depth with a zillion passes of a single-point hook cutter was another matter.

Furthermore, the "spirit" of the BP muzzle-loader was still hanging about. In the classic P-53 Enfield, the bullet, developed from the French Minie Ball, was BORE sized at the point of loading and tamping down on the powder charge. The pressure from the black powder ignition pushed into the large hollow base and expanded the soft LEAD skirt into the grooves. This expansion served two purposes; it sealed the bore and thus enabled more efficient use of the propellant gases AND, as each bullet whizzed up the barrel, the expanded skirt scraped out some of the fouling from the previous shot.

The bullets designed for the .303 in MILITARY service all have open bases. The jacket is relatively thin and soft-ish, the inner lead core is very soft. So, when you kick the bullet in the rear with twenty thousand PSI, not only will the bullet be persuaded to move forward, it will tend to bulge a little at the rear. So, the front half of the bullet "rides" the lands whilst the very rear expands a few thou and acts as a gas-seal / piston. The fast burning rate and consequent "early" peak in the pressure curve of Cordite help in this process.

HOWEVER, Mk7 ball or a close commercial facsimile are somewhat elusive commodities, so most reloaders use a bullet of approximate weight and hope for the best.

Sadly, MOST of the commercial bullet offerings are WRONG for this cartridge and rifle combination. Closed-end, flat-base bullets tend to not "bump-up" to the same extent, even with relatively fast-burning powders. Boat-tailed SP / HP bullets are even worse, as the boat-tail tends to divert the propellant gases around the bullet and into the tiny gap between the bullet body and the major diameter of the grooves. This high-pressure gas acts as a cushion against bullet expansion at peak pressure and also causes gas-cutting in the barrel throat / leade.