The GrayHawk is an old technology rifle but a good one. All I have ever heard from owners is how good they shoot. If I remember, they had a 1-48 twist and used a #11 nipple. Any stainless steel hot shot type nipple with 1/4-28 threads should fit that rifle.

With that twist it should shoot most anything. I would start with 80 grains of Pyrodex RS and some .490 roundball in a pillow tick patch with a good lube. I would also try some maxiball, and even some 240 grain XTP sabots. I am just guessing here and going by the twist. Maybe a Grayhawk owner will pick up on this.

Good luck with your new rifle.