The STI unit is similar to the Model 1 unit from Marvel, which is the more accurate of the two. It has been somewhat improved in that the STI conversion has a last-round hold open whereas the original Marvel does not (though the current models might, I'm not really sure about that one). This unit will fit a single-stack gun although you will have to get a new magazine for it. The magazine is the only difference between the double stack & single stack units.