There are quite a few options on the market for mid cap magazines. Questions on feed and fitment are always a concern though, no longer. The G&P High RPS magazines are nylon fiber mid cap magazines that work in virtually any airsoft M4/STANAG base gun you can think of!
They're what I've personally used for the past 4 years or so now and have yet to have an issue with them. They've kept up with DSG builds I've done for customers and are easy to maintain.
Compatibility: Valken Battle Machines, G&G Combar Machines, Krytac MK1 & MK2, VFC M4s, VFC/CA/DBoys SCARs, Tippmann M4s, and many more!
Note: While they do fit in the VFC 416; inserting them is kind of awkward. I recommend sanding them down for easier magazine insertion.