You're correct about the 45/70. With the changes in powder and projectiles, it's not your grandaddy's rifle anymore.
I think you're a little confused by the .50 though. The modern .50 caliber sniper rifle is far and away more powerful than anything they had in the 1800's. Today's .50 is a .50BMG caliber. It stands for Browning Machine gun. The cartridge is the same which is still used in the M2 machine gun.
The Standard load in the Civil War was a muzzle loaded .58 caliber Minie bullet with about 70 grains of blackpowder.
After the Civil War, the 50-70 cartridge was accepted followed by the 45-70 during the 1870's.
The "Trapdoor Springfield" was available in both 45-70 and 50-70.
The Remington Rolling Block was also available in 44-77, 44-90, 50-70, and later, the 45-70.. ( a very popular caliber of the times.. )
Of course, the most famous of the buffalo guns was the Sharps Model 1874..
Sharps also made a 50-90 model, used by famous buffalo hunter Billy Dixon who took out an Indian warrior at 1,500 yards. (.85225 miles )
Other makers were Ballard, Maynard, and Spencer. ( which was also considered and exceptional gun..)
------------------------------------------------------
I have read an article (or ad) recently concerning 45/70 and 45/110 lever-actions..
They are readily available in the modern market..
Reason for interest was the recent talk of "outlawing" 50 cal rifles in CA..
I figured one way to get around it was to simply go to the 45/70..
I am sure that the 50 cal of today is better than that of yesteryear..
That, however, wasn't the point.. In fact, it proves my point..
If a powder and ball 50 cal. ( or 45/70) was powerful enough for Bison and Grizzly in the 1860's and 70's, today's models are more than sufficient..
And then the 45-90 and 50-90. I think that is about as far as it went.