We tested 5 ML's of different kinds about 2 years ago, just to see whether they would shoot the advertised 150 grains of powder, with any kind of real accuracy. We found that NONE of the rifles would shoot 150 grains at 100 yards without spreading a 3 shot group a good 8 inches. When we backed the rifles down to 130 grains, they started to produce groups of about 5 inches. When we backed the load down to 100 grains, the groups finally got down to 3 inches, but our best groups with all 5 rifles came at the 90 grain load, with good solid 1" groups and some of them were clover leafs.
We were using Pyrodex select, with 240 grain bullets, with Hornady .44 magnum hollow point, pistol bullets and MMP short black sabots HPH 12 I think was the number. Two of the rifles were centerfire ML's with shotgun primers, and three of them were sidelocks, and percussion cap #11 fired. All the rifles had a 1 in 48 twist.
We used loose powder, that was measured very carefully, to get as good a comparison as possible, and used a lead sled to take away as much shooter problems as possible. All of the rifles were fixed with 4X scopes of various kinds that had been sighted in at 25 yards using 90 grains of powder as a starting place.
Rifles were spit cleaned between rounds, and let cool so we would not get any heat problems with the sabots. When we shot these rifles the weather was around 55 degrees and absolutely no wind at all.
We only tested the Hornady bullet, and did not fire any round ball or maxi's as we figured the heavy loads would produce more blow-by than the sabots did. The sabots started loosing petals at the 110 grain mark, and had real problems after 120 grains(probably why we could not get any accuracy after that point)
Conclusion: There are probably some advertised rifles that will actually shoot 150 grains accurately, but we didn't own one. All of the guys that were there are still shooting 90 grains and haven't changed a thing.
Hope this helps.