The best extension - Killer Innovations Velocity Glock 17 Magazine Extension

These extensions are great. Many colors to pick from also. The build quality is superb also. I would highly recommend them over all others. It says it can’t be used in extended magazines. But I cut a small piece of wood to add to the bottom of the spring as to raise it up a bit to its original height. So it’s basically for looks and does not add any additional rounds to my 33 round Glock magazines.