Take it easy Frank I did mention your name and it was not you I was specificaly making referance too but just a generalized statement in trying to be helpful to others. This is just to let the group know what my real intent was and not what you percieved it to be by sharing all of your demeaning statements trying to prove it as being otherwise.

Keep up the good shooting now lets get back to helping this gentleman as was requested.
JLouis

Marlin's or Dale Reynold's  (they worked together on them and I don't know who actually did the work on them--but I suspect it was Dale) was the one I was referring to. They only made two s far as I know.
 Incredibly creative, ingenious, and effective, but a total PITA to actually make.

I suspect that it would have to be custom made to work with each specific rifle given all the gaps and angles involved--at the least that lower engaging lip would have to be.  The BSA martinis might be more dimensionally standardized, maybe the Swiss Hammereli's too, but the German Martinis had so many variations and builders that thy'd be a problem

that the type that will make use of and extended breech bolt that sticks out on both sides in some cases the shat of the bolt can be counter drilled on each end and threaded for a finale that the claws on the breechseater can engage.   Some of the non-BSA Martinis already have a large head on one end and the other has a little snap catch.  that can be replaced with a D/T head that matches the original. 
  I have a couple BSAs---but one is a 22 rf and the other is a varmint  rifle for fixed ammo so I have never really looked at them for adapting for Breech seating. 
I'm assuming the Bass/Reynolds seater method would work--it the gaps can be accommodated, and someone can be found to build and fit it.
A properly shaped push seater would as well---with its inefficiencies. 
If the pivot pin can be modified with relative ease the McCluny seater might be the best way to go.

As on the cadets, the breechblock pivot pin is not visible, the breechseater illustrated wouldn't work.  However, I do believe that David Kaiser was shooting a cadet action which he breechseated, using a modified locking pin for the breechseater to work on (the pin keeping the mechanism in the receiver, at the lower front of the action)

P1,   if I am understanding your query correctly,  I would suggest that if the bullet enters the barrel, whether being fired from the case, or from being breech seated, incorrectly it will probably also exit the muzzle incorrectly. 
Except in a situation where a lead bullet is soft enough to consistently "fire-form" with the powder-ignition pressure front and bore friction to assume the correct alignment and shape before exiting; I don't see how an improperly throated bullet can be "corrected" in the bore.
so I guess my answer it that proper exit is predicated on proper entrance----but JMHO.

Crickets

G,day
       In reply o the OP ?, I sent him photos of my seater ,he made a copy ,said it works fine.

    Cheers Mal in au.