Brian, I've shot the Short Dot on 14.5 M4s, 16-inch recces, and an 18-inch go-fast gun that belongs to SFC Joe H. The two shorter carbines used the Larue SPR-E mount, but I can't remember what the mounts were on the 18. Joe put 45-degree add-on rails and a JP short range sight on the 18, so that when you tilt the rifle to the left you get a post and U-notch arrangement about where your left hand is holding the fore-end. When the rifle is straight up-and-down your close-range irons are at 2 o'clock.
Mark Larue will cut you a cop deal on the SPR-E at SHOT if you ask him nice.
