Early in the SPR (Special Purpose Receiver) program they used full auto M16/M4 trigger groups (auto, not burst). I've shot one of those, and it was kinda meh, honestly.
Somewhere later in the program the R changed from Receiver to Rifle, and they were built with civilian 2 stage semi trigger parts. A Mk12 Mod 1 is late enough it probably was a semi-auto.