Greetings My Dear Yahoody -
whilst I cannot speak for USFA ( in fact I cannot speak for anyone but myself, lol ) I can offer the meager data Ihave collected in my notes over the course of several lustrum ....
Colt Single Action Barrels Thread
pre-1976: 11/16-20TPI
1976-Current:(3rd gen) thread was changed from 11/16-20TPI to 11/16-24TPI.
11/16"-20 tpi fits intermediate and late Uberti,
11/16"-24 tpi fits Ruger Single Action;
.695"-20 tpi fits 1st and 2nd generation Colts, plus Armi San Marcos and EMF Hartford, .695"-24 tpi fits 3rd generation Colt.
---
Pistol:
SPECS: High speed steel.
Colt Single Action, OLD .695" - 20 thread, 1-½" dia. die.
Colt Single Action, NEW 11/16 -24 thread (0.6875)
Smith & Wesson
J Frame, .500"-36 threads. 1" dia. die.
K Frame, .540"-36 threads. 1" dia. die.
L Frame, .562"-36 thread, 1-½" dia. die. ( 9/16)
N Frame, .670"-36 thread, 1-½" dia. die.
Ruger Single Action (centerfire),
.680" - 24 thread, 1-½" dia. die. (11/16 = .6875)
GP-100,
.625"-24 = 5/8-24 thread, 1-½" dia. die.
Redhawk (all models):
.750 "-20 = 3/4-20, 1-½" dia. die.
"Speed Six"
.556"-24 tpi (9/16 = .562)
Virginian Dragoon , 3/4 x 18 ? use 3/4 x 18 and take off .03, clean up with tightened die?
.715 maj .663 minor
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
hope this helps
prof marbels