I think we need to clarify a bit..., a .50 or a .54 will kill whitetails out beyond 100 yards with a broadside shot to the lungs..., and even out to 150 yards if one is using say a 90 grain powder load...
The rifles that we use are accurate out to that distance.
The problem is..., is the shooter capable of sighting the rifle on to the deer's vital areas at ranges between 120-150 yards using iron sights?
I have 20:18 vision..., and last year I shot a large doe at between 100-110 yards, through and through, with my .530 patched round ball, launched by 70 grains of BP. (I thought she was smaller, and was closer). The ball went right where I aimed it..., and it was 9 o'clock in the morning, she was West of me, so was in full sunlight. I was able to steady the rifle against a tree trunk, so for me it was like shooting from a bench. Now at 150 yards..., it's very tough for my eyes to center the sights just behind the deer's elbow on the lung area...,
Now deer don't always make morning appointments on sunny days.., we see lots of them shot in the dim light of the dawn, or the fading lite at sunset or just after. Days are often overcast, or foggy, or drizzling/snowy, and the bright light of an open field on the best of days is different from the criss crossing shadows of the woodland. At 150 yards in shadow..., I can see the movement of the deer and the white tails, but the gray of the deer against the gray of trunks of the trees..., not able to pick a spot to place my shot.
So for the original thread poster..., why do you have your heart set that the rifle must be always used in excess of 100 yards out to 150? OR is it that you'd like a rifle that can reach out to 150 yards, but most of your shots, realistically, will be at 100 yards or less??
I'd suggest you get your.50 with a slow, patched round ball twist, with good sights, perhaps with a flip up tang peep sight in addition to iron sights fitted on the barrel. Using a stout powder load, practice out to 120 yards, and flip up the peep for the extra long +120 yard shots. I don't think you will be disappointed.
Note however, when you go to a peep sight, it reduces the light coming into the eye, and acts as a sort of lens, and makes shots in low light difficult for most people.
LD