I recently bought several Walther P88 (full-size) mags. I commented on how they looked somewhat similar to Beretta mags, and the shop owner replied that the Beretta 9mm and Walther 9mm mags are interchangeable, but one has to cut/move the mag catch cut-out to make them work in the other brand.
I have read and seen posts elsewhere where in it was said that the full size Walther P88 mags will fit in the P99 and latch properly, but will stick out 1/4" or so (note that the P88 compact, at 14 rds, will not seat full inside the P99, so can't be used). So the inference is that Beretta mags will work in a P99, but may need the mag catch cut-out notch modified.
I have a 92 and P88, and I can slide the mags inside the other pistol. For those who tried to insert the Beretta mag into the P99, did it go in but not latch?
If one was attempting to get hi-cap P99 mags, getting the Beretta mags to work vs the P88 mags (if it can be done) is the way to go. Beretta hi-caps are PLENTIFUL, and about half as expensive as the P88 mags are going for now.
Regards,