White Bear,
Making a chambered breech plug like you mentioned in your original note is no big deal.
I've used the same sort of thing on a couple of my guns .. helps to set the lock ( in my case Mule ear and underhammer ) back a bit more towards the back of the barrel..
I set n witness the plug and then position the percussion cone ( or a bolster ) and drill a shallow pilot hole, just a bit into the plug .. Take the plug back out and drill the face of the plug to create the 'chamber' and stopping it to interesect with the other.. cone the face of the plug a bit and polish it all up..
Reinstall the plug to the witness mark.. finish drill and tap appropriately!! A chambered. Plug can and will jam a cleaning jag if you are ham fisted, when cleaning.. and some feel they are harder to get clean?? Which is why I said to polish it inside
other than some extra steps,, no big deal???
Oh one caveat!!!! Make sure your chambered area holds less powder than your lowest charge you anticipate shooting!! Mine were made to hold about 25 ish grains.. that is in a couple of .45 Cal barrels one one .50 cal
Respect Always
Metalshaper\Jonathan