16 rounds. That 17th one must be tight.
Book says 16 but I can fit 17 in mine. Is this supposed to be a 16rnd or a 17 rnd mag?
16 and don't even try to shoot with 17 in the mag....
Put in the amount that the mag was designed for. If you're using the gun for self-defense, do you want to risk a mechanical failure when your life could depend on it? My mini van seats 8, but I could get 9 or 10 in there, but I don't because that's not what it was designed for. Just because you can, doesn't mean you should.
As a n00b at the time, I discovered at the range I could put 16 in my Beretta mag (which is supposed to hold 15) thinking I was super smart or got a 'facotry freak' mag or something. Put it in the gun and I couldn't get the slide to pull back. Tried to pop the mag out and it got stuck. Ok, now what? I can't remember what I did but I was able to get the mag out with my fingertips or something. I'll never do that again... I felt like such a tool.
Better put the amount designed for that mag. Any more could lead to a stoppage.
fyi, promag makes a 32 rounder for the xd9 if you're really wanting hi cap.
I have 3 mags for my XD9I just checkedand 1 of them will hold 17 without extra effort. There is not quite as much play when you push down as the others with 16, but it fits in the gun, and feeds when I operate the slide. When I get to the range I'll try it out, however When I carry it I never put in more than 15, 14 +1, anyway.