What is the familial relationship between the sig lowers? I have a SIG SPEER LT with a 16 inch barrel. I saw Garand Thumbs video on the PDW/SBR version, the Rattler. Would the lower from the SPEER LT be able to take a rattler upper? I get lost on the generations of the Sig piston guns. MCX, LT, blah, blah.