never thought of that busta. Plus I had a machinist friend look into it and he said you would have to remove some of the face of the breech plug to allow the nipple to sit in far enough to let the action close. By removing the metal, it will make the plug weaker and possibly dangerous. So, I would not recommend anyone trying this.
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Dont be so quick to dismiss an idea that may have some merit. Any
issue of strength reduction requires an analysis of the stresses
placed during firing vs the capability of the design. An altering
of the design does not mean absolutely that the capacity
will be reduced below the stress level minus a safety margin.
Remember that in the muzzle loader almost zero thrust forces
are applied to the action at the primer location. The breech plug
itself is absorbing most of the thrust forces. With the use of
a 25 ACP case for instance with and proper head space, some
of the rear thrust could be transmitted to the case head with
zero stress occuring in that portion of the breech plug.