With a McGowen 26" varmint barrel, I had .25 to .333 inch groups at 100 with 140 Hornady Match bullets and H4350 powder. I am still working on Amax 140's. They are 'spose to be better yet.

Those will feed fine with the short action mag. If you want em much longer, you might need the L/A and its mag. Won't matter if you single-load, though. A nice option for single loading is SSS single load thing-a-ma-jiggy some people call the "Fred Sled". But you don't NEED it, just put a cartridge on top of the mag lips and it should go.

I do not know how the two compare in barrel life. Won't know about my own for a while, either. :)

Fire forming is nothing. Just load 260's rather hot and shoot them. They hit close to where they will once fire-formed and you've refined the load. In the long run you will save on brass (longer life) and you will find there's no stretch, so there's no need to trim. I am using Lee Collet Neck Sizing dies, and I don't need to lube, either. It's not perfect, but neither am I. Some day I may graduate to better dies, but I doubt it. :)

Honestly, I think every 260 should be AI. Maybe everything else, for that matter. But I really don't know about others.