While I am FAR from being knowledgeable on the subject, I do have a FA model 83 in 454.
I am still experimenting, but this is what I have found so far:
21 or 22 grains of AA9 behind 250 gr Hornady XTP (not XTP mag). Managable recoil, good for farm pests and deer size game. Do not push this load over 1500 fps. This is a fun load to shoot and is economical. The 250 grain XTPs are not expensive at all, and can be found most anywhere. The Wal-Mart here has them.
36 to 39 grains of H-110 behind 240 gr Hornady XTP mag. This one is a hoot to shoot! Hand the gun to a friend to shoot and see the look in his eyes! If you are recoil sensitive, this is not the load for you. I took a Whitetail doe with this load last year and it works. I thought that while shooting game in the woods, I wouldn't notice the sound. Wrong! This load hurt my ears.
21 to 23 grains of AA9 behind Penn Bullets lead 270 grain Thunderhead. This load is fun to plink with. Recoil is managable. I would imagine that this would be good for deer size game.
31 grains of H-110 behind Penn Bullets lead 270 grain Thunderhead. This is another one that is not for the recoil sensitive. Definitely good for deer and other farm pests.
I cannot comment on accuracy as I only shoot iron sights off hand. I am still getting used to this hand cannon. So far, the latter load shows the most promise as I have been able to contain a 25 yard 10 shot group enough that I can cover it with my fist, about 2.5 x 4 inches.
All of these loads were taken either directly from the FA book that come with my gun, from speaking with FA representatives, or from Penn Bullets.
As a note: I am not sure about other brands, but the FA 454 has a twist rate of 1 in 24. If you slow the velocity to much less than 1400 fps, accuracy will suffer. These guns were meant to squirt bullets out fast, up to 2000 fps with a 240 grain XTP mag bullet. High velocity is where these guns shine.