Martin- the muzle crown is very important to the ease of loading and being able to use an appropriate thickness of patch. You can do as Tinker suggests, using a tapered object and gringing compound, or the easier method of your thumb and emery cloth - I use 320 grit.

Even if the crowns are cut with an object, they still should have a smoothing with emery to cut any sharp edges between the angles that are produced.

When shooting off bags with a gun that is recoil sensitive- ie: throws shots if not held the same, I hold the gun as if shooting offhand, and rest the back of my left hand on the bags. The left hand grips the forend or barrels the same each time.