I just recently bought one of these 1851's after getting a 5 and 1/2 inch barrel Remington. I have fired 30 grains of Pyro-P in the Remington and it is SWEET.
I wouldn't use that charge in the 1851 brasser.
The flyer that came with my 1851 (I got the round barrel version in .44 cal.) denotes no more than 25 grains of fffg blackpowder due to the brass frame.
I would use slightly less in 777 equiv. as it is a little hotter. Pyro-P is volumetric and should be O.K.
From what I've read from others who own brass frame guns, mild charges = long frame life and better accuracy, so... moderation is the key.
If I want to get funky and make loud noise and mucho smoke, I stuff the Remington and let her rip. That gun can take it.
The 1851 I recieved from Cabela's is very nice, well made, tight, indexes well and has good balance.
All my buds who've seen and held this gun like it!
All in all, I am pleased!