M4, AR15, M16 are essentially the same gun and will fit the same mags.
There is a STANAG agreement on wepaons magazines for NATO which is why so many Euro guns take the M4/M16/AR15 mags and vice versa.
That is why HK G36 mags will also fit M4/AR15/M16
The HK416 are ever so slighlty different in the magwell and will accept most but not all AR15 mags..
Basically the TAPCOS and PMags and USGI fit any Ar15 M4gery etc that you can buy.
As do any mags you have left over from the last deployment.