Today, the surplus firearms market is saturated with these firearms, which are typically on display by the case at local gun stores. Their caliber (close to .308) allows them to be used for cheap hunting rifles, and they are pretty accurate, reliable, and tough.
The challenge with these firearms is that the 3/8 sight rail is a bit anemic for mounting a scope. A common problem with them is for the sight ring screws to loosen up after a few rounds (even with locktite).
This instructable documents my 3rd (yes third) adventure in mounting a scope to my Mosin Nagant M91/30.
Note: This documents me making permanent modifications to a firearm. I am NOT a professional gunsmith and am not responsible in any way if you make modifications to your firearms. If you are not comfortable with the modifications in this instructable either structurally or for retaining historical value...don't do it! If you perform this modification...IT'S ON YOU !
Step 1: Required Hardware
(4) Metric #4-.70x12 Cap Screws
(4) Metric #4 Wave Lock Washers
(4) Metric #4 Split Lock Washers
(2) 10-32x1/4 Set Screw
(1) 1/16 Allen Wrench (or whatever fits the set screw).
(1) Tube of locktite.
(1) 10-32 tap
(1) #21 Drill bit
(1) pair of medium height 3/8 dovetail scope rings (I used the cheap NCStar ones off Amazon).
With the sight rings, but not including the scope, I have about $15 in the mod.
Step 2: Drill a Hole in the Scope Ring Bases
Step 3: Dimple the Rifle
If you value the historical accuracy or doubt the structural integrity of this mod, then don't do this step!
Dimple the 3/8 sight rail of the Mosin Nagant where the set screw will be tightened. I've assumed the rear iron sight has been removed.
* Position the sight rings on the rail where you want them and snug the mounting screws. Helps to have the scope on hand to mock up where you want the rings.
* Use the same drill bit used for step 2 and a hand drill to drill down through the hole in the sight ring base and "dimple" the sight rail on the gun.
* The dimple should only be as deep as the head of the drill bit (just barely - see picture).
Step 4: Tap the Holes
Run the 10-32 tap all the way through the hole. These aluminum scope rings were easily tapped with the tap mounted in a drill.
Step 5: Add the Set Screw
Step 6: Add the Sight Rings
1. Inch the newly added set screw out slightly.
2. Position the sight rings in the right spot so that the set screw points "fall" into the dimples. Keep in mind which ring goes to the front and which goes to the back.
3. Add locktite to each side mounting screw and thread them back into the sight rings.
4. Tighten the side mounting screws to their final tension (set screws should be completely over the dimples). If sight rings are aluminum and threaded to accept the side mounting screws, be careful not to tighten so hard you strip the threads.
5. Tighten the set screw to it's final tension using the allen wrench through the top of the base.
Step 7: Add Scope
Your done. Go sight it in. This is my 3rd attempt to mount a sight to this gun. I ran 13 rounds through the gun after this and it was still rock solid.