All you need to do is eliminte the action.
Bore the barrel a bit and rechamber.
Drill out the primer pocket of your 25-06 brass and replace the primer with
a piece of burn through film.
Add a rolling block type breech to your barrel.

Add a cap lock to accept home made nitro caps and you are good to go.

When they outlaw nitro swap out for a flintlock.

When they outlaw flintlock, just swap out for a pan and douse with
home made black powder and a match lock.

Pssssssssssssssssssssssssst --------- Boom