Most of the rest or your question has been answered but i would like to add...try the 410gr 50cal Great Plain IF you intend to use it on larger game.

It wouldn't be my first choice but i find them on close out for under $8 fairly often. They are a flat point but still have the HB and "might" work better. They are no longer offered by Hornady but they show up occasionally. My LGS got in several boxes and i bought all of them for $6.95 per 20.

I agree with Ron on the Partition at its current price or for almost any price if i got a chance to hunt elk. It is a far superior bullet and its nearly impossible for it to fail/fragment or fail to expand. IMO the Lehigh 270gr should work well too and they offer sample packs for about $12.

Ive used the 300gr SST on deer with no problems but i was driving them a little harder than your load. I was even using the red low drag sabots but in a semi tight bore. IF i was going to use a SST/FTX on elk it would be the 325gr .458 made for 45/70s. IMO it holds up better than any other SST/FTX ive tested on media.

The FPB is basically a copper plated hollow base conical with a plastic tip. The 350gr MIGHT be ok for cow but on a big bull i would have some concerns about a humane kill if the shot is a bit off.

When hunting my number 1 priority is the most humane kill possible and the price of a box of bullets i need to get it done is insignificant to me. They must be within my accuracy requirements and designed for the job even if the shot is a little off. The Partition is going to be a very hard choice to beat even in the 260gr if the shot is good.

If you're using a Great Plains with a 1-66 twist, i really have no idea what would work best, all of mine have been 1-38 or faster. I prefer faster twist rates for sabots and most conicals. Im assuming you only meant the bullet and they have been fine on deer sized game with about the same load.