Thank you everyone! I'll be following in Archilochus' footsteps and omit the flat.

From what I can see of the mechanism, the length of the barrel isn't a really critical dimension. It looks like it could tolerate even as much as +-0.125" Yes? (aside from legality of course). I can see a faint wear mark where the original barrel rested and I'm going to make mine reach to roughly the same mark.