Long range .303

[Patrick Chadwick]

the entire inventory of machine-gums

? Is that some kind of Aussie tree? With clockwork koalas eating the metal-foil leaves?

But seriously, thanks to Bruce for an excellent explanation! To which I would like to add: since the cartridge must have some sideways play in the chamber, the case neck will actually be sitting on the bottom of the neck portion of the chamber. So the gas flow around the bullet is not symmetrical. Result: the bullet is pushed to one side, which will cause it to be engraved with a slight skew.

Before anyone laughs, the amount of skew to produce 1 MOA deviation over the 1/2" cylindrical section of a typical bullet is about 1.5 ten-thousandths of an inch. So when the bullet emerges from the muzzle it is being launched with a small precession that will increase the effect described by Bruce. Somewhere in his books Dr Beat Kneubuehl has a picture showing how precession and nutation can produce a small spiral deviation in the trajectory that worsens the group size as it is a matter of chance where the bullet is in the spiral movement when it hits the target.

BritishBeer, you could try the Hornady type 3130, 180 round-nose, with probably the longest cylindrical section you can get in commercial bullets. They are at least a partial cure if your rifle has a worn throat. Alternatives are the Speer 180gn round-nose and the Sierra 180gn ProHunter spitzer.

And, of course, make sure that you use neck-sized cases for reloading. You don't even need a neck-sizing die, just back off the normal sizing die so that a little unsized ring (1-2mm) is left at the neck/shoulder junction. This un-resized ring will help to keep the cartridge neck centered in the chamber. It's kinder to the brass than full resizing too!

Finally, check that the muzzle crown is absolutely as perfect as you can get it. Any asymmetry (or, heaven forbid, nicks) results in a little kick to one side when the bullet emerges, which also produces the precession/nutation effects.