Judson:
You are quite correct about the vented breech-face NOT catering for a blown primer. They were developed to prevent gas-cutting of the breech-face in the event of a leaking primer. I can look it up in Crudgington & Baker if anyone is interested, but the several variations of the vented breech-face are covered under a Scott patent I believe.

As previously stated, the problem with many shotguns is that the firing-pins or strikers are quite large compared with bolt-guns and nitro double rifles, so a large portion of the centre of the primer is unsupported at the moment of ignition: not a problem with low-pressure shotshells. However at NE pressures, this may cause the primer to flow back into the firing-pin hole, and rupture when the elastic limit is reached. The full pressure of firing is then funnelled straight back into the action and stock-head, with potentially disasterous results.

The best way to guard against this, other than bushing the strikers or firing-pins, is to choose actions for conversion which have small-diameter firing-pins and minimum gap around the pins in the breech-face. Judson, we could probably come up with some guidance or 'rule of thumb' for this. I'll measure my Greener as Tinker suggests, and a few nitro doubles and old shotguns, what are your thoughts on the actual maximum recommended size?