You’ve shot the one in those pictures. Is your memory fading? I’ve had RRA AR platform SBR’s for years. You forgot the Vector SBR. The HK53 copy with the 9” barrel. Once I get past Christmas, I may try and get that “Beretta space gun” added to the SBR list.
I’d love to have a Sig P556 Classic upper to Form-1 an SBR. I have an extra lower. All I’d have to two is pull two pins like an AR and pin it on.