Scope mounting: Should I Lap or Bed my Mounting System?
By Jim See
The picture is of a very expensive action and a bolted and pinned on rail from the same company. On top of it is a precision ground parallel.
You can clearly see at the back of the rail there is a small gap between parallel and rail where the rail drops away from the parallel.
We can assume from this there is some crown in the rail. I could have the most perfectly machined rings and the rail is what would create the misalignment.
Even the best rails can have crown or cup, depending on the manufacturing process, fixturing, and or heat treat cycle.
The exception to the rule would be an action machined with an integral rail post heat treat. These are not given the opportunity to "warp" because they are typically cut in one of the last operations of the receiver, and have already been heat treated to final hardness.
So what measures do we take to assure our system is flat and parallel to best align the rings with the scope tube?
We can get out a surface block and some measuring tools, or a CMM, or optical comparator.
Or we can set our scope in the rings and see that it moves freely on the bottom halves and call it good enough.
Or we can charge a lapping bar and give it a run over and see where contact is and is not being made, for some this may be the easiest option. BUT, I don't think it is the smartest in every instance.
First off let’s talk about the worst case scenario, a rem 700 with an aluminum rail. That rail is going to twist to conform to the receiver 95% of the time. The easy solution: epoxy bed the rail to the action to minimize the twist created by a receiver that was hand polished and does not have a concentric profile. I would also bed a steel rail. (Side note: Always check the length of the front base screw for length so it does not bottom out on the barrel tenon threads in the bottom of the scope base screw hole. I've only seen that at least a dozen times. lol)
We can then mount our rings when the rail mounting process is complete. Drop the scope in it and slide it back and forth a bit to see if it binds. Or slides freely. Depending on what it does I will finish the process in one of two ways.
Binding is a sure sign that tightening the rings will produce ring marks in the scope where an un-concentric edge digs into the scope tube under compression. Under this circumstance I will lap the rings, to give clearance to the ring to scope fit. I would never consider lapped rings as ready to precision mount a scope in. This is why.
Did you confirm the dimension of the lap and the charged abrasive? If you have lapped a dozen sets of rings I'll bet the lap varies in diameter from end to end by at least .010" So you can see lapping may or may not add any precision "fit" to the end product.
So now you must ask, "Well WTF Jim why did you lap the rings then you just said it's not a precision fit"
My goal was to create clearance. The clearance is needed so I know the scope will set all the way down in the rings and not "bind" on the tube.
Once this is accomplished I am going to 2 part epoxy bed the scope into the bottom ring halves. It's pretty easy: clean the rings. Mix up a small amount of 5 min. JB weld. Spray the scope tube with release agent. Put a thin coat of epoxy on the bottom ring halves only. Set the scope in the rings, put the ring tops on and only finger tighten them to feel a tiny bit of resistance.
We want the epoxy to form a gasket just like on an engine head, perfect 100% contact.
Let the small amount of squeeze out stay put. Let the epoxy rest for 20 minutes, take the ring tops off, pull the scope out and then use an Exacto knife to easily cut the ribbon of squeeze out away from the ring.
Clean your scope tube off and install as normal for final mounting of the optic.
If you do this you will have the perfect fit. Is it necessary for every installation? NO. Do I consider it mandatory for some? YES.
You will be surprised how nice of a fit it creates and the gasket acts as a no slip surface between the rings and tube. You will never have to tighten rings up to more than 15 inch pounds after this process on even the heaviest recoiling rifles, it works that well.
For you guys with Spuhr mounts, have you ever noticed how slippery the scope is in the anodized rings before torquing the screws? This epoxy gasket eliminates that.
The fact is after I started epoxy bedding my scopes to rings, I do it in every set-up I shoot. I feel that strongly about the positive effects of a process that literally only adds 5-10 minutes to the time you spend mounting the scope, minus the dry time.
ABOUT THE AUTHOR
Jim See currently competes as a Pro in the Precision Rifle Series, having finished the last 4 seasons ranked in the top 15. Jim has worked in the precision bolt action rifle industry since 2007 as owner of Center Shot Rifles, Quality Manager at Surgeon Rifles action division, and currently operates Elite Accuracy Training and Gunsmithing Business in Decorah, Iowa.