Hi lj,
The later MGC magazines will slip into the TK M4 mag'well, but, the magazine catch would need a bit of work to hold the mag in place. The MGC mag is a little loose in the well too.
I bought a plastic 40 rd mag' intended for real AR15, M16 & M4s sometime ago. Good news is it fits and clips securely in the TK.
As it is though, the bolt / carrier won't strip cartridges from the mag'. I guess that with a little reworking of the feedlips it could be possible to make it work.
The Bolt hold open / release catch won't work either due to feedlips fouling it.
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.