My first go at reloading for .303 Brit

[Patrick Chadwick]

Many Enfields that have seen a lot of use are a little worn or corroded where the bullet enters the barrel.

You can make that "Many old service rifles" - it's not just an Enfield problem.
As a simple rule, I establish the OAL that results in the bullet just touching the lands, then reduce that by 0.5mm (20 thou) as a practical repeatable value, where inevitable production variations will not cause wide pressure and muzzle velocity variations. Using RCBS competition dies, excellent Lapua bullets, and quality Geco cases with a weight spread of less than 1 gn, measured velocity 5 meters from the muzzle is 833 meters/sec +/- 5 meters/sec for my 30-06 ammo. I have not yet done the measurements for 303. Don't try to get much closer to the lands - if you do, the tiny variations in OAL will then cause a wider velocity spread.

Having worked out this optimum OAL, you then have to check whether such a cartridge would actually fit into the magazine! For some rifles, the answer is no, and you just have to shorten the OAL until the cartridges fit - or use the rifle as a single loader (which is what one usually does here in competition shooting). You also have to make sure that there is still enough of the cylindrical section of the bullet (about 1 caliber length is minimum) in the case neck for secure seating. As a result, you may well find that your optimum OAL is only feasible with flat-base bullets, even flat-base round-nose bullets, and boattails are ruled out for the optimum OAL.

Patrick