It’s important to know if you are intending this load for a rifle or a pistol? If you’re at 2600 FPS and looking for more, I’d guess this is NOT a classic Remington XP100 pistol, but rather a rifle chambered in .221 Fireball?
The XP100 pistol with the original 10.5 inch barrel generally maxed out at 2600 of so FPS with most loads. Rifle loads and some custom pistols with longer barrels can be pushed harder.
For rifle loads, H335 is the only published load I can find that just barely clears 3000 fps at maximum recommended loading with a 50 grain bullet. The Hodgedon manual claims several powders will do it with a 45 grainer, but again just barely.
Personally, I’d start working upward from what you’re shooting now and see if accuracy starts slipping. I rarely get the best accuracy at or near max.
I always wanted a XP100 in Fireball, but in the end versatility won out and I got a .223 barrel in Super 14 for my Contender. With a sweet 4×12 Burris that baby has rifle accuracy out to the point where the shorter barrel starts to show the lower velocity. Still very amusing in the p-dog towns.
Grouse