It's pretty simple. A bar of aluminum that's been machined to sit on the top of the gun, bolted in place, and the scope rings tighten on to it. Sound's simple right? Wrong.
The problem we have here, is failure, to manufacture well. ha. The Mosin Nagant was a mass produced rifle in the times of war, so the objective was quantity, not quality. So when you look at the receiver of your rifle and where the mount is to attach on, it is no where near true to the bore of the rifle.
There are few hexagonal receiver Mosins out there, and the chances are the one you have is round, like mine. And when it comes to backyard machine work, a round, out of round, receiver does not lead to quality precision work. Trying to drill and tap this mount into place is beyond the skills of your average backyard machinist, and will not work well.
Once I had mine drilled and tapped into place and what I thought was straight and true, place the scope on it revealed otherwise. I was so far to the right that I had not enough adjustment in the scope to pull the cross hairs back to center. Even though the mount was flush on the receiver, it can't help the gun that it's sitting on.
Knowing that this was a one time deal, I had to come up with another angle of attack. I took off the ATI scope mount, and started from scratch.
I mounted the gun in my 4 way vice, mounted the vice on my drill press, and inserted a mill bit into the drill press. I turned my drill press into a milling machine, and milled a true flat spot on the side of the gun.
After this, I drilled and tapped one bolt into the angle, to the gun.
Then I re-machined the ATI mount, to make it a flat bar, removing the round hollow on the bottom of it.
I drilled and tapped the first hole in the ATI onto the top of the 90 degree aluminum angle. Then I attached my scope to the mount, centered the adjustments on the scope, and inserted the Laser Bore.
After lining up he scope to the little red dot, I drilled and tapped the second bolt into the side of the 90 degree angle. This fixed my vertical cross hairs. A quick realigning, and I took off the scope to drill and tap the second bolt through the ATI into the 90 degree angle, which fixed the horizontal cross hairs.
The scope was now 98% on target of the Laser Bore sighting.
So that's my review. Though the ATI seems like a simple drill and tap fix to your scope less Mosin, it is not, and money should be better spent on a better, newer gun.
Below are two videos, one showing my ATI scope, and one showing the refurbished ATI with 90 degree aluminum angle.