FYI, there are 16 round and 15 round 9mm walther mags, the 15 rounders seem to have anti-tilt followers, but I've yet to have any issues with ammo feeding out of any of them.
Just keep them well oiled/rust proofed and clean them before you put them up if they get sweaty, cause they seem to rust fairly easily.
The Walther P99 AS is a pretty sweet gun. Mine has probably 3k rounds through it and still looks and functions like new, except for a very small amount of attractive holster wear. Mine's been highly reliable after the breakin period. Enjoy.