I have a 1952 LongBranch that I have shot for ten years now. I have settled on 180 grn Speer's over a 44.0 grn charge of AA2520. She clocks 2560 fps and shoot a 1-1.25 inch group @ 100 yards like clock work. For all these years she still shows no pressure signs and so far the best shot I've made is two bullets into the head of a turkey, one at 325 yards, the second at 278 (measured) yards. She's a real shooter
. The load is directly from page 499 of Modern Reloading by Richard Lee. I and my sons have shot hundreds of rounds at this load level from the gun. We've taken deer, hogs, squirrels, and targets without killing our shoulders.
I would recommend you slug your bore, order the right diameter bullet, and start re-loading. It is well worth the effort. The .303 is every bit the equal of the 308's and 30-06's. It (in a bolt gun comparison) exceeds the American rifles. It does have it's draw backs, the action is not as strong as the Mauser based bolt guns. On the other hand if you keep your loads within SAAMI specs no worries. They are accurate, reliable, adequate, and produce moderate to low recoil, that's plenty good for most folks.