Dear Nick;

Since you are a rifle builder and understand metal work and the strength of metals, your question about the removal of the significant amount of metal from the face of a boxlock action to install two-piece retrofit striker discs and what effect it would have on the strength of that action is prudent and indicates to me that you have given it considerable thought. All boxlock actions are not built of the size and strength of the famous Greener Empire action--which has a thick standing breech portion of its action from which the strikers (firing pins) operate and protrude through to fire the cartridge. The Greener Empire action is just about the ideal rugged shotgun action, and of course many would argue that the Webley Screw Grip made the strongest of all actions for a double rifle and just as many would argue that the Rigby rising bite action and the Westley Richards double rifle action were the strongest.

Some (maybe better said, most) boxlock shotgun actions (without factory striker discs) have a significant amount of the back of the standing breech cut away to allow the tumbler with its integral striker nose to freely move forward and strike the cartridge as the below diagram shows (see the link to the diagram). On boxlock action such as this the removal of significantly more metal on the face of the standing breech to add a 2-piece retrofit striker disc assembly is not a method that I find prudent.

Boxlock and sidelock actions that were originally made from the factory with striker discs do not have this significant carving away of the back of the standing breech of the action. If you will scroll down to the second diagram of the below link you will see a sidelock back action shotgun that although it does not have striker discs it shows how thick the back of the standing breech is without the cutting away for a tumbler with an integral firing pin. I will search the web and see if I can find a diagram of an action with striker discs that I can add to this post that better illustrates the issue at hand.

Re-directing primer rupture gas from the face of the action via a slot across the face has been done for decades and seems effective to my mind.

https://www.shootinguk.co.uk/shooting/start-shooting/the-anatomy-of-a-shotgun-46268

Kindest Regards;
Stephen Howell