Dave...you are correct it would be easier to shorten an existing gun to 4 5/8" the problem of course being finding one... There are tons of 4 5/8" guns out there in blue steel and as far as I know putting a Bisley gripframe on one is no major problem... Best place to ask is over on the SingleAction Forums...those guys do it all the time...
mirage...try Jack Huntington at JRH Precision... I've had two guns modified by him and he is one of the masters... And again if you ask over on the SA Forums those guys know everything about SA mods and who does the best job.
...and nice guns bartman...
Bob
