Gas Blowback Rifle Conversion Kit- Angry Gun, HAO, Maddog, and RA TECH are providing the kind of Conversion Kit such as the HK416D kit, John Wick M4 Rifle Kit, Noveske N4 Rifle Kit.
Best frame there is at the pricepoint. It fits all tm-spec just fine, and the only difference compared to an advanced GSB frame is that the housing unit is just a little bit more wobbly