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These disks are meant to guard against primer setback on the face.

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Squirl:
Not sure what you mean by this, further explanation would help.
It is my understanding that disc-set strikers are a very sensible way to fit replaceable firing-pins to hammerless guns, especially side-locks, with the added feature of a 'valve' to prevent gas entering the stock-head through the action in the (unlikely) event of a pierced primer. If this ever happens, the stock will usually be a write-off, and in the case of a box-lock a large sliver of wood could puncture the shooters eyes or face.

I have witnessed this with an FN SxS shotgun: luckily the right side blew off and I found the chunk of stock-wood some 20 metres away!

For shotgun conversions, as others have said, it is to permit a reduction in diameter of the firing pin itself, to prevent primer-cup flow-back into an oversize pin-hole in the breech-face. Maybe the primer would then rupture, maybe not, but it would almost certainly lock the action closed!

Small firing-pins are a must, and low-pressure cartridges with tough primers help guard against this risk: I use Remington 9 1/2 M instead of Federal 215s in my un-bushed shotgun conversion, just in case! Never had a problem....

My .577 Light Nitro conversion:



Bush this baby!!!

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