The hold should be the same as shooting a revolver 2 handed - or cylinder flash will be the least of the problem - spitting lead will make you bleed. Too, the harder, slow burning powder granuals can also imbed in the wrist and arm.

The cap and ball revolving carbines (18"?) didn't have a fore end regardless. Both hands of the grip, the left wrapped around the right for right handed, was the norm for shooting them. Always wanted one, they looked pretty cool! Never got around to shooting one myself.