To all,
   Just curious as to which BUIS sits lower on the upper receiver.  I'm looking for scope clearance on a SPR project.

Thanks,
  VSP

The KAC 600m is definitely just a bit shorter than the ARMS #40.

If you go with the SPR rail used on the Model O, the rear irons hts. will be about the same for optic clearance. You get more ability to bring the optic to the rear for short eye relief optics if on the SPR rail with a #40, because the #40 is shorter and con-cave at it's front. That allows the SPR rail to be able to extend more to the rear, thus the added rear notch space.
Good shootin, Jack

First lets make believe the Scope is a 3.5-10X Leupold on your SPR.

You want the ARMS 35 QD scope mount.  Trust me on this.

You cant use the ARMS 40.  Tried it, it wont fit.  The GG&G A2 is a tight fit and actually touches with Medium height rings.  The KAC 600 fits with room to spare.  The Medium height rings are actually a bit too high by like .125" and if you go with Low rings on the ARMS QD 35 you will have a perfect cheek weld but you can only use the KAC rear sights.

Eye relief is not an issue as the scope has a ton of eye relief even at 10X.  I reccomend the KAC rear sight, ARMS QD 35 with low rings and the 22 rail up top for a later Dr Optic.  This only applies for the 3.5-10X Leupold as a larger objective wont fit with low rings.

Thanks for the info.

VSP