G2 and standard Contender barrels are compatible.
The buttplate on my Contender carbine says, "not legal with a barrel less than 16" attached" or something close. I have both pistol and carbine stocks for that one as well as a spare 10 22lr bull barrel.
Contender and Encore barrels are not interchangeable.
Contenders handle pressures up to and including 357 maximum, the whisper series, and others but not the big rifle calibers. Contenders in 375 JDJ have taken every game animal on the planet.
There are at least three variations on the Contender frame having to do with the break open release...location of the hinge pin on the trigger guard. The oldest ones take the most effort and the latest the least.
I have had a few barrels that did not interchange without some judicious filing. Forends also may need work, especially if using an old forend on a later pistol.
I'm Contender poor: G2 Carbine with 45/70 and 50X209 barrels, Contender carbine with 223 barrel, and late model contender with (2) 22 lr barrels, 222 barrel, 357, 30-30, 45/410, 45Colt and 44mag/hotshot barrels. Thinking about a 300 whisper barrel next.
