Which barrel length do you have? The T/C Thunderhawk came in 21", and 24" barrels, in both .50, and .54 caliber. All have a 1 in 38" twist. I have Thunderhawks in both calibers, with 21" barrels. My .50 likes a 350 grain T/C Maxi-Hunter, with 90 grains of Goex FFG. I get decent grouping with this load, and hunt with it. If you are sighting in, be sure to let the barrel cool between shots. I find that as the barrel heats up, accuracy diminishes with my Thunderhawk. As you are using Goex, a # 11 cap should be fine, but if you prefer, you can also use a musket cap nipple. I switched to musket caps on both of mine. No other modification needed, just screw in the musket cap nipple. Also, the T/C manual for the Thunderhawk says maximum powder charge for .50, 350/370 grain Maxi-Ball/Hunter = 100 grains FFG. Maximum for sabots = 100 grains FFG. I really like these ML's, as they are light weight, and easy to handle when hunting. I disassemble mine completely to clean. If you have not done this before, it might seem challenging, and time consuming, but after you have done it a few times, it is very easy, and it takes no longer, than when I clean my T/C Triumph.