Bought the G19 spring to go with the +5 mag extension after installation of both I can only fit 19 rounds in the mag with proper cycling. It can fit 20, but it is very, very tight. Had to use a mag loader to push it in, but 20 makes it hard for the magazine to seat properly and cycling a round is a bit rough. Overall I am happy with the extension and the way it looks. 19+1 is still a great improvement, but was hoping for 20+1.