Jethro and Rusty are correct. Always find out what the guy is selling that is being billed as an "expert"( that includes me! I am not selling anything, pride myself in basing any opinions on observed facts, have actual experience in doing original research and publishing an article in a peer reviewed magazine, and have no problem limiting my opinions.) I don't have much experience shooting the .54 cal. rifles. I have shot some loaned to me on the range by friends, but that is the extent of it. However, that .54 caliber round ball weighs 230 grains, and is larger in diameter than most modern " bullets " get to after then expand in an animal. It delivers a long, deep, primary wound channel, with some secondary tissue rupturing just from the vacuum begind that wide ball that drags air into the primary wound as it is created. It may be only going 800 feet per second at 100 yds, but it kills because of the huge hole it puts in things. A moose is not that hard an animal to kill. I would not hesitate to use a .54 cal. rifle on Moose out to 100 ydas. However, if you ask me the same question about using it on Alaskan bears, I would cut the maximum range substantially. I would also recommend using a .58, .62, .69, or .75 caliber rifle for bears. With those heavy skinned and boned animals you want the heavier weight of those larger round balls to smash through the animal and destroy vital organs. The .58 weighs about 5/8 oz; the .62 weighs 3/4 oz; the .69 weighs 1 oz, and the .75 weighs 1 1/8 oz! And remember that they are soft lead balls that also expand when they hit flesh and bone.
A .62 RB will exit the other side of a deer leaving an .80 caliber hole. That larger than the diameter of a quarter. Different rules apply in determining killing ability with these large soft projectiles than with the modern, high velocity, small diameter, jacketed wonder bullets. Both have their place; both kill, but for different reasons.
My 6.5 x 55 mm swedish mauser kills so well because it shoots a small diameter, but very long bullet, with a high ballistics Coefficient, and pretty fair sectional density. That means it keeps on going. At high speed, it drives deep, even in heavy boned animals, and its secondary wound channel is awesome. That is the one created by hydrostatic shock, caused by two shock waves, one off the nose of the bullet as it passes, and the second off the base. Both of these waves, distrupt tissues, and cause hemorrhaging around the primary wound channel for several inches. The bullets kill not because of a massive primary wound, but because so much tissue, and internal organs are damaged by the secondary wound channel.
When dealing with the writing " experts " read a lot of material, and study, study, study. The small bore vs big more fight has been going on since the 19th century brought us smokeless powder. If you let them, there are lots of fellows who will blow smoke UP your tail pipe over this subject. It pours over into questions like this, with someone invariably recommending some high speed, smaller bored load, every time, over a PRB, simply because the guy has never really spent any time using a PRB or understanding how it works to kill so well.
If you are raised on smokeless powder ballistics, ( as I was) and were taught to believe that only a small caliber high speed bullet is humane to use to hunt game animals, ( I was told it but didn't believe it) you are going to put down PRB muzzle loaders any chance you get. I have a rather disjointed education in rifles. I went from shooting a .22 rifle, to shooting my first high power rifle, which was a .45-70 trapdoor Springfield with 500 grain builets and semi-smokeless powder loads! I was about 12. The idea that those slugs could not kill anything that roams N. America, was ridiculous. I later learned the same about large caliber round balls. My first Black powder pistol was a .62 cal. smoothbore flintlock! That " pumpkin ball " did not move very fast, but did it move things it hit!