From my experience ,shooting 150 grains of powder in my muzzleloader is to much.I lose accuracy and bullet performance. 90-10 grains is much better.I shoot powerbelts as they are legal at home and Colorado where I hunt.
I think that a lot of that magnum load crap was marketing. I shoot 90 grains of Alliant Black MZ and it works fine. A lot of that 150-grain charge just makes more noise. Muzzle loaders are suppose to be for close ranges and 80-100 grains works with about every bullet.
I also shoot powerbelts. I shot 150 grains of triple seven for quite awhile and after some research I decided to try 100 grains. At 100 yards it still shoots nearly the same POI and also similar sized groups. All while being safer, easier to shoot and cheaper.
My groups got better when I dropped down to 100 grains of triple 7 with my 295 grain powerbelts. With the extra powder, the gun with only burn so much and blow the excess out the end of the barrel, so it will be different amounts each time, therefore, the different groups with the 150 grains. Stick to what shoots best.
Back when I used to own an Encore I decided to try the 150 gr. charge, split the stock and had to buy a synthetic replacement. I'm with ElkStalker, 90 gr of RS and a .530 PRB is all that has been needed to put the meat up.
Every muzzel loader is different, but I can’t imagine one being efficient with a 150 grain charge.
When I worked up a load, I started at 90 grains, and work up to 120.
Accuracy dropped off, velocity gains diminished and recoil skyrocketed above 115, so I backed it down to 110.