If you go thru the process to get the Federal "Permit" for a SBR you can put a stock on a modern revolver. However one issue with the old Colt Revolving Rifles was that if you used the forearm you could end up with your sleeve on fire. With a modern Magnum if you were wearing a short sleeved shirt I suspect a pretty bad burn could result.

BTW, perhaps I'm mistaken but I thought the exemption for Antiques actually ONLY applied to antiques made BEFORE 1898, not replica's or more recent "antiques". If I am wrong about that I'd be interested in seeing a clarification.