If you are building a 380 gun, go with the small dedicated M11A1 .380 magwell, the original M11 380 design, not the “large grip” SWD 9mm size magwell. The larger magwell (the one on that 11/9 blueprint) is the same for the M10/9, M11/9 and the SWD “large grip” M11A1 .380 (zytels). The availability of the original modified Zytel 380 mags (marked 380 on the spine) is very scarce, they are basically collectibles at this point. They were just normal 9mm zytels that were heated and squeezed shut slightly at the feedlips to prevent the 380 rounds from popping out like they do with the normal 9mm zytel mags. The only alternative now is installing steel feedlip inserts to use 380 with the standard 9mm zytels, which is easy but can be tedious every time you get a new magazine. Like Strobro said, the ZMAGS don’t work as well with 380.
Regardless, the small steel mags work much more reliably and the profile of the gun is much sleeker and smaller with the original 380 small magwell. Can’t comment on the reliability of the FTF small grip 380 steel mags, I’ve had a few of them pass through, some have plastic followers (I think they use steel now) very rough finish on them. The old factory CMP/RPB cobray marked mags work great, they’re always available on gunbroker, but they can be a little on the costly side.
