First off welcome to the forum. It is great having you here.
You bought a great rifle, but I think they are know for their sabot shooting ability more then their conical shooting ability. Although I think you are on the right track. I have no personal loads to share with you, but I do have some suggestions you might consider.
If the powerbelts are working for you, then stick with them. Try some of the different weights and designs. Aero Tips, and some of the pure lead ones might work real good for you. I like to shoot big conicals, but I have one rifle that loves the 245 grains Aero Tip Powerbelts. With 90 grains of powder, there is nothing safe for 100 yards with the open sights.
Also some other things to try would be some Saber Tooth Conicals. They shoot well out of my Black Diamond XR and my Knight rifles love the things. The Saber Tooth come in 250, 300, & 350 grains. My rifles like the 300 grains ones. They are very similar to the Powerbelts in design, but I like the way the bullet is made better, and they are a lot cheaper then Powerbelts.
Instead of Maxi Hunters, try some 370 gr Maxi Ball. I never found a rifle yet that will shoot maxi hunters. Why I have no reason.Contact
http://blackjackhill.com/zencart/index.php?main_page=product_info&cPath=1_7& ;products_id=27 Get a hold of Bill Davis at Black Jack Hill and have him cast you some up out of pure lead. You have to lube them yourself, so you might need a tube of bore butter. They normally shoot very well out of most rifles and have excellent down range power.
The last option I would suggest is find out what the barrel of the rifle (not the QLA mind you) slugs out at. Then have some conicals made at Bull Shop Conicals. I shoot a .503 out of my Black Diamond XR with excellent results. I am going to get some .502 diameter next time and see how my Knights like them. I shoot them with only 70 grains of powder, but when the conical weighs 500 grains, that is a lot of power. Hop that charge a little and elk might have a real bad day with that load.
Another thing to examine is your selection of powder. While I do not want to start a powder war here, I never had any luck shooting conicals with American Pioneer Powder. Get some Triple Se7en 3f powder. I like that the best for all conical shooting I do. Also the normal Triple Se7en 2f does well, I just up the grains charge about 7 grains and it works good. For example, I shoot a 500 grain conical with 70 grains of the 3f, and 80 grains of the 2f. Both hit the same place. Both have excellent power.
Another good powder for conicals is Swiss 3f, or Goex 3f. The thing to remember is black powder is filthy. I makes a lot of smoke and stink when you shoot it. I mean, what more could a person ask for? Swab the barrel between shots also, when working up a load. It will help tighten your groups.
The thing I want to say is, there is really no way anyone can tell you what will shoot well out of your rifle. All rifles are a mystery to themselves. I have a number of Hawkins and Renegade with the same twist barrel. One shoots conicals perfect, the next one would not hit a barn door with them. All you can do it try.