Hi - I'm new here, but certainly not new to hunting Moose, Elk or Deer with large bore rifle ctg. guns, modern rifles or ML rifles.
: I don't think there is any "BEST" ctg. for these large animals. I would prefer something in .40 cal or above, with 300gr. or more of lead. The ctg. should handle 65gr. of BP minimum. A .45/60 might be just fine, close-in, as most bush shooting happens to be for say, less than 100yds. shooting. 40-50's are for deer at best, but probably great for that.
: That said, the 45-70 does a good job as always, and the .50's are even better with 70 to 100gr. powder.
: The 3-1/4" cases are very difficult to get to shoot well when shot dirty - that is, without having to wipe the bore between shots.
: Paul Mathew's books on loading BP ctg. rifles are excellent and almost manditory for good results.
: His recommendations on lube and methods of loading turned my .45 3-1/4 into a MOA rifle with it's iron sights. The methods work with all ctgs. no matter what bore size or case length.
: I'd hold case length to 2 7/8" max in .45 and 2 1/2" in .50. More powder isn't needed nor recommended due to increased recoil for very little gain, if any.
: Most .50's have too slow a twist for good shooting - a 24" to 28" twist is about as slow as works really well - that is, getting good accuracy in the sub - 2" range.
; My "Optimum" BP ctg. would be a .50/90 with 24" twsit using the Luman 520gr. or RCBS 550gr. grooved lubricated slugs. Our .50 Alaskan (2.1" .348 case) Bro's Sharps and my RB made consistant 10" and better groups at 450yds. with these bullets in McGowen 24" twist barrels.(shot off the hood of a truck)
: I hope this helps with your decision on an Elk or Moose ctg.