YMMV but I use FFFg in my .54 rifle and .62 smoothbore with no ill effects. I've used equal volume charges of FFg in the .62, but find the Tulle fouls less, is easier to load, and quicker to clean when using the finer powder. Good hunting!

I prefer triple F in my .50, use the same load, the 3F might be a little quicker. They are both the same powder, just a qround a little smaller, so you will get a little more powder in the same volume.

For most practical hunting loads, you may not see much of a difference, using the same loads measured by volume. If you are wanting to fine tune the load, use the recovered patch method, get a real good feel for the FFg, then match the patch with FFFg loads. Bring the FFg loads up to the point of burn thru and burn past, match the effort with FFFg and look at the difference. The load reduction for matching results will probably be close to 5-8 grains or less. FFFg brings with it smoother flowing, and easier cleaning, less large fouling. I tried using FFFg in my 54 about 30 years ago, never went back to FFg. Real black powder, right? Black powder shooting is all about having fun, have a bunch!

tanoose,
dont worry about the velocity.i have a .4o,.45, and two .5o cal rifles.all i have ever used is 3f bp and spit patched round balls. .my one .5o load is 60 grains.start with 50 grains and work up till you find what charge your gun likes.large charges dont kill deer, hitting where your aiming is what kills.
good luck