Full capacity magazines will absolutely fit and function 100%! However, if you insert a 17 round magazine into your "CA legal" Glock 17 within the borders of California, the first thing that will happen is the gun will malfunction by failing to strip a round into the chamber. As long as the magazine is in the gun, even if you single-load a shell and drop the slide on it, the gun will fired only once and the slide will lock back. At the same time as you insert the 17 round magazine, your location will be instantly recorded and transmitted via sat-feed to local police, the highway patrol, ICE, and BATFE. Your location will be visually recorded using an NSA "keyhole" type satellite which can read the words on a dime from 150 miles up, even through clouds and smoke thanks to quantum thermal imagery. At this point attempting to escape is futile as the LEOs will be converging on your position within moments and will have exact images of you, your car, everyone with you, the gun you are holding, and of course, your location to within a meter. An FBI file will be automatically created on you, your entire family, friends and co-workers. Go ahead, ask me "how" I know all this.... My advice; if you live in California, do NOT bring any "banned" capacity magazine within six inches of a firearm in which it could function. Just don't do it.