I’ve got my pair mounted on 19.5s and they do fine on a pistol of that size. Plenty of good work has been done with an Acro P2 which has a smaller overall window size than the SCS (Not comparing anything about the two other than the objective, measurable size do the glass) I’ve said before that I wish the window was taller as I don’t think a slightly taller window would detract from concealment.