Re: RUM based longrange caliber choice.
As far as the "best" big 338, or any other bullet diameter, the grass is always greener on the other side no matter which side youre on. Each has different yet unique characteristics. One has to sort through these differences and decide which one is "best" for his ideals.
As far as getting a 338/300 RUM not feeding smoothly in a 700 LA is hog slop. It takes a shorter thoat if you dont have Wyatt box mag and a few less grains of powder and a FEW fps. With the 300 grain BC's you can sacrifice a few FPS. If you get the Wyatts magazine you can throat it a bit longer and seat the bullets to nearly 3.8" long and still have reliable and smooth feeding
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.