The advice I got from an old craftsman at Cogswells when they were in Piccadilly is that the flats should clear the smoke.
Re the forces generated and how a double shotgun receiver reacts to them, it seems that the phenomena are a little more complex than the old notion of the cartridge acting on the breech face.
1-There is no mention of the Poisson effect on thick walled cylinders, and the chamber end of a barrel is definitely a thick walled cylinder.
2- Slamfire DIY shotguns are a totally unlocked system. In these the barrel and "breech" tend to recoil as one unit. It seems that the cartridge adhesion to chamber walls is strong enough to make both act as one unit in recoil.
3- Judging by barrel imprints on breech faces the above seems to be a factor we have yet to analyse.
4- Small Bore has written about firing a double totally unlocked, with no ill effects. Gough Thomas writes about a double firing with nothing but two layers of scotch tape holding it shut. If you measure the locking bolt of a pump or auto it is puny by comparison to the locking system of a double. As far as I know autos and pumps do not suffer from loose lock up or coming off the face. That should tell us something about the forces acting on the bolt.
