Here's some info on 150 gr. in Muzzies.
"No .50 caliber inline can fully burn more than 130 volumetric grains or so of blackpowder or Pyrodex. I've not tested every gun on the market, but even the tightest grouping guns lose their accuracy after 120 or so grain charges, depending on the specific projectile. The most experienced muzzleloading hunters and shooters on the continent, including one gentleman who has some 80 muzzleloading elk among his substantial credits, have documented this. Those elk all fell to bullets propelled by 120 grain (or lighter) charges."
http://www.chuckhawks.com/magnum_muzzleloading.htm
But then again other's swear by it!
"I shot a Hornady 250gr XTP mag with 150 grains of 777 and I thing it is a great load. Ballistically speaking, Im around 2200fps at the muzzle and about 2300lbs at the muzzle with 2 inch groups and 100 yards."
http://www.realtree.com/forums/showthread.php?t=2181
There are soooooo many conflicting stories on the use of "150 grains" but from all I've heard I'm sticking with about 95-120 gr.
max. Different weapons have different pressure ratings, and it would be wise to look into that at least. Safety thingy....
Of course, every weapon is different and everyone has ways of determining their own road to accuracy, safety, and reliability.
To achieve "magnum" status, an in-line has to be able to
handle a powder charge of 150 grains of propellant. (but not all weapons are classified as "capable", or a better term is "safe", to do this of course.) The gun manufacturer will have info regarding this, as each ML is different.
