Just wanted to give a preliminary report on using Beretta 92F mags in a P99. I have one in a Walther P99 right now, and it appears to function fine so far. I haven't test fired it yet, but will report back when I do. Here's what I did:
1) Remove slide
2) Insert 92F mag until it *almost* touches the stop
3) Scribe mag body along edge of mag catch (looking down into mag well from top)
4) Dremel new notch
I have to clean the cut up when I get a small file, but so far so good. When the big pot-metal follower is in the 92F mag, it doesn't always hold the slide back with no round in it. With a follower from a 10 rd P99 mag, it seems to work fine :)
The mag sticks out of the grip a little bit, but I'm thinking it will be fine for at least range use. I will update when I test fire it. That probably won't be until next weekend, but perhaps I'll go out this weekend.
Edited to add:
PS - The mags I have are pre-ban Asian contract 92F mags (made in Taiwan). With the P99 follower in it, the mag holds 17 rds. The 92F spring is a little wide for the P99 follower, so I just squeezed the spring shut a little with some pliers.
Edited to add pic :)