I've hunted with muzzleloaders for 51 years and have taken 20 elk in the last 40 years of hunting the critters: 3 with ctg guns (7 x 57, 338-06, 45-70), and the rest with round balls. I've also been in on the shooting of maybe another 100 elk or so (both as a guide and a partner - about 75 were with muzzleloaders). I have seen elk taken with everything from a 257 Savage, 30-30, and on up to a 470 Nitro and a 45-120 Sharps in ctg guns and in muzzleloaders from 45 Express (i.e. conicals) up to an 8 bore.
1 elk was with an 11 gauge original Potsdam musket - a rear end going away shot on a big bull (6-700 pounds) at 90 yards after the hunter I was guiding made a bad shot - the ball went from one end to the other - load was a 585 grn ball with 120 grns of 2F.
8 cows and 2 spike bulls (for myself I don't hunt antlers only meat) with a 54 which is my favorite caliber overall (partly since I can use the same balls in my 28 bore smoothie) - my loads were .526 ball or .530 ball with 85-100 grns 3F (different loads for 3 different rifles)
Finally what the OP asked about - 4 cows and 2 bulls with a 50 caliber - load .490 ball with 80-90 grns of 3F.
All but one were one shot kills, none over 75 yards, and the longest run (after a heart shot) was 125 yards. Most bit the dust within 50 yards at the most. On the other hand I also turned down plenty of shots when I knew better than chance a wounded critter - but I do that whether I'm hunting with a muzzleloader or a centerfire.
So yep based on my own experience as well as that of several others, a 50 caliber will do the job as long as you do yours and nope you don't need super heavy charges or big heavy conicals to do it with either - just know your gun and yourself and as the man once said your "limitations" - put a ball into the boiler room and you will put meat on the table.
Ever experience a muzzle loader used to kill a moose?????or bear??
Speret - never shot a moose but have seen a few taken with muzzleloaders mostly with 54-58's plus a couple of 62's and 69's - all with round ball. As for black bear, many moons ago I hunted them with a passion when I lived out in the Pacific Northwest. I took 30 of them with muzzleloaders - again most with my 54's, but two with my Potsdam 11 bore, five with my 50, four with my Manton 16 bore, and two with my Rigby 45 Express. I used the same loads as for elk and again knew my limitations - on bear no shots were taken over 40 yards, most from ground blinds. They ranged in size from 150 pounds to 400 pounds with most around 200-225.
Only had to do one follow up and that's when a friend had an unfortunate bad shot (critter took a step just as he fired and instead of a chest shot he hit the paunch - bear weighed about 250). He ran off and we let him hole up for a while before going into the bush after him. He made a charge across a small clearing (about 25 yards across) and my buddy hit him once with his 54 - a good, but not stopping shot that turned him towards me and I hit with a 12 bore double (a one two shot) at about 15 yards - hit him good and it was like he ran into a brick wall...those big 500+ grn balls do a good job when it's up close and personal like.