There seems to be a fair amount of confusion about the installation of these mounts (available at Marstar or Frontier for $150), and some disappointment when they don't work right or hold zero. Correct installation, and a bit of improvisation should eliminate all the reported problems, but the mount comes without instructions. So here's my step-by-step that has proven successful (I've edited this post as experience has dictated):
The first step is to remove the dovetailed stripper-clip guide from the receiver. To do this, use a 3/32" punch (the Glock armourer's tool also works great) to remove the hollow retaining pin from the guide, then drift out the guide using a larger punch (1/4") and ball peen hammer, pushing from the right (charging handle) side of the rifle to the left (scope mount) side. Don't use nails or allen keys for this work. You'll bust a finger or scratch your rifle.
Then, tap in the Marstar dovetail anchor piece, having separated it from the rest of the mount. Drift it in to the the receiver from left to right until it is flush and centred. Then crank down the two allen screws that retain it in position (use a little blue loctite - both on the screws and on the dovetail slide itself).
Now try dry-fitting your mount, fastening in the front knob/bolt into the threaded hole in the side of the receiver. You may find that the knob, when fully tightened, does not go in far enough to tighten the mount to the side of the receiver. This is because the receiver is not fully threaded all the way through, possibly to prevent a too-tight bolt from interfering with the action of the bolt. If you encounter this, the solution is simple; remove the bolt/knob from the receiver (it's held in place by a sort of c-clip on the inside), and put a small locking washer in between the knob and the outside of the mount. Put the c-clip back on the inside, and you've not only solved the snugness problem (without forcing the bolt too far into the receiver) but also protected against the knob working loose.
Once the receiver is snugged up against the body with the front bolt (don't crank it tight at this point), take a look at the rear mount. It is what controls the side-to-side alignment of the rail. When the rear bolt/knob is out of the hole, you'll notice that the hole in the middle of the sleeve is hexagonal. this allows the sleeve to be adjusted in and out through use of an allen key. So back off the locking nut on the outside of the receiver, and insert an Allen key, and you'll see how moving the sleeve in and out moves the rear of the mount left to right by pushing against the dovetail anchor you've installed in the receiver.
So how do you check the alignment of the scope mount during this process? I used a rail-mounted laser (pulled the TLR-2 off my Sig), and put it on the mount upside down. You can then line up the laser dot with either a laser bore-sight or (more practically) with the iron sights. That is, adjust the mount left to right until the laser dot is vertically aligned with the iron sights' point of aim. When it is, tighten the locking nut on the sleeve, put the rear knob/bolt in the hole, and snug it down, tightening also the front knob/bolt. If you don't have a rail-mounted laser, you'll have improvise with a dowel or even use a mounted scope to align the crosshairs with the point of aim of the iron sights as you make the adjustments.
Remember, the scope mount will not appear to align flush with the side of the receiver, because the side of the receiver is bevelled (apart from the area surrounding the mount hole). The mount should stand vertical. So don't chisel or dremel away at the guide ridges on the mount to make it fit more snugly. If it's installed correctly, you should be able to use the iron sights underneath the rail, and the sight picture should be centred.
When you've done all this, mount your scope to the rail and adjust for comfort. Then snug it down, using blue Loctite on the scope mounts. When it's secure, check the alignment with a bore sighter. Then, dismount the scope, and reassemble using Loctite on all the screws, and also put some (or other epoxy) along the mounting ridges on the receiver to ensure a snug fit. Check alignment again using a bore mounted laser (if you have one) and your scope.
This should give you a rock solid, perfectly aligned mount for the optics of your choice.
The first step is to remove the dovetailed stripper-clip guide from the receiver. To do this, use a 3/32" punch (the Glock armourer's tool also works great) to remove the hollow retaining pin from the guide, then drift out the guide using a larger punch (1/4") and ball peen hammer, pushing from the right (charging handle) side of the rifle to the left (scope mount) side. Don't use nails or allen keys for this work. You'll bust a finger or scratch your rifle.
Then, tap in the Marstar dovetail anchor piece, having separated it from the rest of the mount. Drift it in to the the receiver from left to right until it is flush and centred. Then crank down the two allen screws that retain it in position (use a little blue loctite - both on the screws and on the dovetail slide itself).
Now try dry-fitting your mount, fastening in the front knob/bolt into the threaded hole in the side of the receiver. You may find that the knob, when fully tightened, does not go in far enough to tighten the mount to the side of the receiver. This is because the receiver is not fully threaded all the way through, possibly to prevent a too-tight bolt from interfering with the action of the bolt. If you encounter this, the solution is simple; remove the bolt/knob from the receiver (it's held in place by a sort of c-clip on the inside), and put a small locking washer in between the knob and the outside of the mount. Put the c-clip back on the inside, and you've not only solved the snugness problem (without forcing the bolt too far into the receiver) but also protected against the knob working loose.
Once the receiver is snugged up against the body with the front bolt (don't crank it tight at this point), take a look at the rear mount. It is what controls the side-to-side alignment of the rail. When the rear bolt/knob is out of the hole, you'll notice that the hole in the middle of the sleeve is hexagonal. this allows the sleeve to be adjusted in and out through use of an allen key. So back off the locking nut on the outside of the receiver, and insert an Allen key, and you'll see how moving the sleeve in and out moves the rear of the mount left to right by pushing against the dovetail anchor you've installed in the receiver.
So how do you check the alignment of the scope mount during this process? I used a rail-mounted laser (pulled the TLR-2 off my Sig), and put it on the mount upside down. You can then line up the laser dot with either a laser bore-sight or (more practically) with the iron sights. That is, adjust the mount left to right until the laser dot is vertically aligned with the iron sights' point of aim. When it is, tighten the locking nut on the sleeve, put the rear knob/bolt in the hole, and snug it down, tightening also the front knob/bolt. If you don't have a rail-mounted laser, you'll have improvise with a dowel or even use a mounted scope to align the crosshairs with the point of aim of the iron sights as you make the adjustments.
Remember, the scope mount will not appear to align flush with the side of the receiver, because the side of the receiver is bevelled (apart from the area surrounding the mount hole). The mount should stand vertical. So don't chisel or dremel away at the guide ridges on the mount to make it fit more snugly. If it's installed correctly, you should be able to use the iron sights underneath the rail, and the sight picture should be centred.
When you've done all this, mount your scope to the rail and adjust for comfort. Then snug it down, using blue Loctite on the scope mounts. When it's secure, check the alignment with a bore sighter. Then, dismount the scope, and reassemble using Loctite on all the screws, and also put some (or other epoxy) along the mounting ridges on the receiver to ensure a snug fit. Check alignment again using a bore mounted laser (if you have one) and your scope.
This should give you a rock solid, perfectly aligned mount for the optics of your choice.
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