I make them Dan, but for other folks. I personally have little use for the 10 bore or bigger. I am not going to Africa again, so I like to use my 62 for everything.

I am 100% satisfied with my 62 and I live in elk/moose country. Elk are common here like white tail deer are most other places. I have killed elk, moose deer and antelope with it.
I made one for a friend a few years ago, and he killed 17 different animals in Africa up to Eland. Worked like a charm.

So I think a 62 is a fine caliber for game up to about 1500 pounds, and can be used on game up to 2000 pounds if you shoot well.
The old "standard" of the British empire (if any could be called standard" was the "1 ounce gun" A 16 bore rifle is a very good all around caliber for large game.
Your 50s and 54s are 100% fine for any deer size game in the world, and plenty of elk are killed here every year with 54s too.

I may someday build a 12 bore for myself, but not for African hunting. Just a good English style big bore, but set up with sights to work with a fairly low velocity load. A 12 bore ball will weight about 500 to 530 grains depending on what size I end up using. That's about 1 1/4 OZ Now if I load to 1200 FPS it's not going to be any different then the old 12 gauge slug loads of the 1960s, but it will be a LOT more accurate.
Should be a wonderful hunting arm. I do enjoy the elegance and handling of the good British style arms of the 1850-1870 era.
I also love the handling of the find German Jaeger rifles.
Now that I think of it, I also love the handling of American Longrifles…..
Heck…….............I like them all.