This problem could vey well be due to a combination of factors, the most significant of which are probably powder type and the .300 RUM's large case volume. It is likely that RL-22 behaves a littleerratically when used in such an overbore case at less than 100% + loading density. Both the .300 RUM and .300 Weatherby just have a bit too much case capacity to work well with some powders. You usually can find one that will do better than a variation as high as 100 FPS between shots, but sometmes it is a real PITA to do so. I have heard of cartridgesbeing so overbore that the barrel was shot out before a good consistent load was found!