[BattleRifleG3]

I think you have an excellent observation there. It would explain why the only modern revolver longarm I've seen is a shotgun, whose shells of a given length would seal the cylinder the same way before being rotated. This would restrict you to a given length of shell for a given shotgun gauge though. As such, the wisest gauge choices would be 28, 16, and 10ga.

So I suppose two different designs would be in order - a Nagant style system for 28, 16, and 10ga shotshells, and an enclosed system for rifles. And the enclosed system, which would be harder to load, would coincidentally have a higher capacity (5 instead of 3 rd).

Gimme a few decades and some of these ideas may become reality.