Actually i talked to Del about the problem and that is why Barnes had him make a different sabot for them.

Ive shot many many 54x50s and the 300-325gr options are PLENTY. The 50-300gr FTX performed really well. The 50-300gr GoldDot performs awesome in the 1700fps range and penetrates nearly 22" of gel. Speer offers a 50-350gr SP if you want even more penetration.

I seriously doubt you will get more fps than i can with 120gr of BH209 in a side lock, so your choices are even more limited. My little GMB54-120 carbine (6.5lbs) with a 385gr CLHP, pus sabot weight and a max load of BH209 is barking right on the heals of a full sized 458 Lott rifle in recoil. The calculator on handload.com puts my top load at over 55lbs of free recoil at just 1750fps with a 385gr/13gr sabot and 120gr of powder. Thats also adjusted for the extra weight of the scope and rings.

Recoil really isnt the problem, its getting the bullet to stabilize well and heavier bullets will need over 1700fps to even get close. Plus they may not expand properly if they will shoot slower.