A customer asked me this the other day and I couldn't come up with a good answer reading the law, and CBI wasn't much help either.

Say two CO residents exchange a long gun in another state?  Do they need to do a FFL transfer when they get back?  I doubt the idiots that wrote the law every really planned for that, but the question came about because these two guys had been hunting in Nebraska and one had bought a rifle from the other at the end of that hunt.  

Federally they weren't in any violation, that much I know.