There's another tool you probably have lying around.
When you use your shotgun to open a door, do not aim at the lock itself; aim for the locking mechanism inside the door. That is, point the barrel at a severe angle to destroy the mechanism, not the door knob.
There was a segment on "Mythbusters" about what type of gun was necessary to destroy padlocks and deadbolts. The fools shot all manner of weapons at the door lock with almost no success (until they got up to the howitzer level). If their goal was to gain entry - instead of destroying locks - they should have tried to shoot THROUGH the door into the mechanical part of the lock instead of the face of the doggone thing.