I agree with DD re: the circlip guns. I have a Mk III, earlier without circlips, as does a friend of mine who did the work, as a barrel changer set. Barrels in 12 ga, 12 bore rifle, 50-90, 375 2 1/2", .303, 45-70, and .22! Very versatile set. I have fired many rounds thru all barrels, with no troubles whatsoever. Bear in mind, the .303, and possibly .375 need to be kept to earlier, and milder, loadings.
Doug