I've got the M261 unit that I put in an SP1 upper since it's a 1-12". I had to do a good bit of tuning to the unit to get it working the way I liked.
A dedicated upper to match the weight and feel of my DCM upper will be in the works soon. It's a real advantage to be able to go indoors and shoot 200 rnds using the same rifle, sights, and position you'd be shooting outdoors.