Here is a way to have the option to reliably use your 10 round 995 carbine mags in your C9. The 10 round carbine mags do lock in to a C9 but due to the excess length that is unsupported, they tend to have some wobble which can cause rounds to nose up resulting in failures. The simple solution is to make a magazine sleeve. I have used the sleeves that come with the Promag 995 15 round mags. Over the years through horse trading and such, I have managed to acquire two of the Promag sleeves. You just measure the gap and cut, then sand to fit. Result is a stable 10 round magazine that you can now swap between the 995 and C9, just remove the sleeve to use in the 995.