Reloading the .303 has its own sets of challenges. I inherited an .303 sporterized Enfield Mark III and learned to reload since factory ammo was very expensive and hard to find. The only inexpensive factory ammo was corrosive surplus (nearly impossible to find) or Prvi Partizan ammo from Serbian (not very accurate in my rifle 4-5 MOA).
The chamber dimensions of the .303 Enfield rifles are much more generous than an unfired factory round compared more modern designs like .308 rifles. I think this was a battlefield hold over from the black powder days as reliability was most important to the British Military. Since the .303 was invented in the black powder era, fouled rifle chambers were a big concern and the .303 Enfield was designed with a very loose chamber so the guns would work in dirty battlefield conditions.
After firing a factory case, the brass expands significantly to the fit the large chamber of the rifle. If you don't neck size the .303 you will experience dangerous case neck separation or at the very least very short case life from stretching the brass back and forth.
.303 uses .311 or .312 size bullets which are more expensive and harder to find (mail order only for me) than .308 bullets which are available everywhere reloading supplies are sold. Reloading the .308 allows you to utilize the latest and greatest bullet designs like all copper hunting bullets or bonded bullets. .311/.312 bullet selection is much more limited.
Ballistically, the .303 (174 grain bullet at 2500 FPS) is probably closer in trajectory to the 7.62x39 (123 grain at 2400 FPS) AK round than the .308 inchester (168 grain at 2650 FPS or 150 grain at 2820 FPS). At longer ranges, the .308 is noticeably flatter. The kinetic energy of the .303 is much closer to the .308 than the 7.62x39.
Overall, I don't see much reason to stick with the .303. Its fun for nostalgia purposes and but I really see no advantage.
Acurate .308 rifles are common and inexpensive nearly every manufacture chambers a rifle in .308. Even entry level rifles are expected to shoot 1.5 MOA or better with the right factory ammo. Surplus .308, while not plentiful, is still readily available. Surplus .303 is nearly non existant outside of some unique places like Pakistan or Afganistan.
Last edited by Flakbait; November 13, 2012 at 02:00 AM.