There's a company that makes a VERY easy to install internal velocity reducer for the TK/KJ M4A1... I'm currently tinkering around with one... It's really fun
Unfortunately as far as I know, no other brand of magazine is compatible with the TK gun
But there are rumors of the same company that makes the aftermarket part.
http://www.cradleairsoft.com/Cradle_Airsoft/Home.html