OK, please first understand there are reasons for asking. Neither are there any laws to be broken.
Can such a pistol be obtained? The specifications needed are:
1) At least .44 Magnum, Can be bigger.
2) Supplied with Buntline length barrel from new.
3) Double action revolver.
4) Here's the catch. The barrel needs to be relatively easy to change from Buntline to, say, 6" and back again by a person with mechanical knowledge and the right tools but who is not necessarily a gunsmith.
5) A handy benefit would be if the thread on the barrel (assuming it's threaded) is common enough that a gunsmith could make the alternative barrel out of a standard barrel blank.