>>64443202
FRT's are generally harder to pull off for straight blowback - the design team behind the CZ Scorpion Evo hit major snags, most strongly recommend delayed AR PCC's instead of blowbacks for FRT's. This leaves the Vz61 and the MAC-10/11 which are SS compatible with a printed lower taking an AR trigger - they need a fair bit of tuning compared to other platforms.
And no the PS90 isn't straight blowback