The reason most powder measures max out at 120 grains is with loose powder, that is a magnum max load. Many times this is or near equal to three 50 grain pellets. Most rifle companies will put a 120 grain loose load as the max limit for their rifles. There is really little need to go over that.

Many large North American game animals are taken with 100-110 grains ofloose powder. If your shots are 150 grains and under I would not concern myself with trying to get the load over 120 grains. If you want to see what magnum loads will do, get some pellets, load three pellets, and in your Truimph try some 250 grain Shockwaves. A lot of the T/C rifles are made to shoot them. And most other rifles shoot them well.