A myth except for the 44-40, 38-40 and 32-20.
5.56
Yes and no. I don't think those were being referenced, just the 45 colt. Anyhow, the 32-20 wasn't around in 60, 66, or 73. It's really the 44-40 that did a lot of the work and then 45LC swooped in and stole all of the glory.
Edit (added): 44-40 beginning in 73. Prior, 44 Henry. But I think it was much more common to have a cap/ball gun converted to 44-40 or 44 colt than 38-40 in the mid to late 1870s.
