Harve,
Even though your early '76 has a dark, rough bore, it just may shoot AOK if the powder gases can be kept behind the bullet while it transverses the barrel.
About 5 years ago, a distant friend and I got together for a shooting vacation. One of the rifles he brought along was an original '76 in .45-60 that had a 32" barrel. Unfortunately, the barrel was the worst I have ever seen....dark, very rough...even the chamber had pits deep enough that fired cases had to be removed with a cleaning rod!
Anyway, a 5 shot group at 25 yards grouped into just under 12"(!!) with all of the bullets printing sideways profiles.
I then tried a .06" poly wad under the bullet and there was a dramatic improvement. This time the 5 shots grouped
into about 4" with the bullet holes cutting fairly round profiles. I then tried two .06" polyethylene wads under the
bullet and those 5 rounds impacted within a 1 1/2" circle.
Since your rifle is a .45-75, I would suggest trying 1.) a 300 gr. bullet with a .06" poly wad so the wad will be retained in the neck, or 2.) the standard 350 gr. bullet with polyethylene shot buffer.....say 60 grs. of b.p. then fill the case to about 1/2 way up the neck with psb and then seat the bullet.
You may be surprised on what your vintage '76 will do with the proper ammunition.
w44wcf