In the context of this thread, (& the SB <18" rule) that would be any change to the gun from it's original factory condition. It would be easy enough to add that pistol grip & cut down the barrel of a model 500, but that would be illegal. Buy it new from the factory in the exact same dimensions, & it is legal. Clear as mud? In case you didn't read my first 2 posts correctly, I'll say it a third time. There are other manufacturers making them.