I'm not going to mention much about the 170yard moose, shot with a .54 except that - perfect hit- broadside, both lungs & heart, ball stopped against the off side hide, load was 100gr. stricken Pyrodex, ie: about 80gr. - TC measure with .535" pure lead ball, .018" ticking patch - moose, a big bull, took off running like they normally do when struck with a small diameter ball - dropping dead mid-stride in 40 yards. He did not make the bush. I see more moose dropping inside 50 yards than past - I see moose not chased, lay down and die in 50 to 75 yards. If int he open, they will try to make the bush and seeming safety - THAT can be a force to keep them on their feet. Both lungs and I doubt a moose will do 100 yards - I've not seen it, except when shot with a .300 or 7mm mag.
BTW- that RB .54 had approximately 210 - maybe 230fpe at impact - not quite the 1,500 fpe the 'books' say. The books are full of nonsense. FPE does not kill animals. FPE does not relate well to large diameter or heavy bullets well - doesn't work much better with small ones either. If it did, a .22-250 and .204 Ruger would be more powerful at 100 yards than a buffalo killing .45/70.
A .54 will work- but as with any gun, you have to hit the animal in the right place. Poor shooting on game is responsible for losing that game, not range or ball size.
Use enough gun - a .54 is enough gun, put it in the right spot and that moose or elk is dead on his feet.
I prefer bigger guns - bigger guns make bigger holes, but a smaller gun with more powder, is better than a bigger gun with a squib load. If you can' t handle the recoil of a .58 to .69 with heavy, flat shooting charge - by all means use a .54 with a descent charge on moose - say 120gr.2F. there is not much recoil in a .54. If you think that kicks - you need to shoot it more. That's a descent load that gives a point blank range of 130 yards with no sight correction for range- easy shooting on moose,w ith a .54- if you can do it and don't get buck fever. If that happens, you need to get out more.