I have a .45 Hunterbolt, which I believe is very similar to the Magbolt. It took me awhile to find what it likes, but now it shoots very good.
First of all, forget about using pellets- you'll need loose powder to get good accuracy. I would suggest Triple Seven, in FFFG form if you can find it. If you can't find FFFG, FFG will work okay too.
I would start out trying the 200 grain SST or the 200 grain Shockwave- both are pretty easy to find. Start out with 80 grains of powder and work your way up in 5 grain increments until you find something that shoots good. In my Hunterbolt, the 200 SST is the most accurate bullet (1.25" at 100 yards) along with 95 grains of powder.
I've also had good success with the 200 grain XTP with the same powder charge.
If you want to use Powerbelts, I was able to get the 225 grain to shoot good (2" groups) using 70 grains of powder. I wouldn't use more than 80 grains of powder with a Powerbelt.
Hope this helps!!!