ALL Lee-Enfield and Lee-Metford rifles were fitted with barrels with 1 turn in 10 inches, which works just peachy-keen fine for any bullet between 150 and 215 grains.
ANY rifle with ENFIELD rifling (odd number of lands and grooves, deep grooves, lands and grooves of equal width) will shoot its best with FLAT-based bullets.
My TEST load for the .303 round is DI brass, CCI or WLR primers, 38 grains of IMR-4895 and a Sierra Pro-Hunter 180 flatbase pointed bullet, seated to the OAL of a Mark VII military round. With this loading I have had .5-MOA groupings from several rifles, smaller in a Ross. This is NOT a heavy load, produces 2250 out of an SMLE, 2335 out of a Ross.
By actual British TEST in 1910 it was determined that MOST Lee-Enfields will shoot their most accurate with a .312" bullet. Only Hornady makes this, but the other American bullets are soft enough that if you kick them hard on the back end, they will obturate. An excellent load for the Hornady 150 flatbase is 40 grains of 4064, bullet seated so the entire cannelure shows. This is a reasonably zippy load but not absolute tops. Much venison in freezers around here from this load...... and it shoots half a minute if you can hold the rifle steady.
Hope his helps.
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