Shade Tree Gunsmithing: bedding a scope base.
One of the things everyone in the precision rifle world deals with is dimensional tolerance. Most of the fixes are well known: actions are trued, match grade barrels installed, high dollar optics mounted, rings lapped, stocks bedded, etc. One thing that has been overlooked for years is the fit of a scope base to its receiver. It may seem odd to think that a scope base may need to be fit to a receiver but it is a fact. All receivers have some dimensional difference on their circumference but round rear bridges (newer Savage or custom like Stiller/Pierce/etc) seem to be better than actions with flatter bridges (Rem 700 or Win 70).The same way that a stock is the platform for the barreled action so the base is the platform for your optical sight. A scope base will benefit from epoxy bedding as much as a rifle stock will.
The idea had never occurred to me until a few years ago. I spent my high-school and college years (late 90s to early 2000′s) working in a gun shop where I mounted hundreds of scopes. Back then nearly everyone used either Leupold or Weaver mounts and often as not they were two piece bases. If you did not have scope alignment bars (almost nobody did back then) you would have no idea that a scope mounting system had issues until you bore-sighted or zeroed and didn’t have adequate elevation or windage adjustment.
Our solution (supported by Brownell’s selling shim packs) was to shim bases to adjust elevation issues and to use Leupold windage adjustable bases (or the dreaded Millett ringsif a customer insisted on something cheap) . Shimming was a viable solution but one I never liked. I knew there had to be a better solution.
During this same period of time I met a guy who was a mechanical engineer and hobbyist gunsmith. He used a reamer to cut his rings into alignment. To me this was a better way to deal with things like uneven bases than shimming was or mounting holes out of alignment by means of windage adjustable bases. It still did not deal with putting things back into alignment though.
The other thing this guy was adamant about was that when you bedded a rifle into a stock that you did it with the scope mounted since there was apt to be stress on your action due to some level of misalignment stress due to your mounting system. I assumed this was probably most important if you did not ream your rings since that should remove all stress but there is a limit to how much you can ream and lap mounts. I still did not have a one-hundred percent solution but what I was beginning to to really put together was to eliminate stress in your system if you want success.
Around the time I was seventeen I began bedding my own rifles because I was not happy with the bedding jobs I had paid for from a couple of gunsmiths. The bedding was functional but they didn’t use pillars and their finish detail was poor; their should not be any unfinished edges to include being able to see where it flowed in the barrel channel because that is half-assed sloppy work. The more bedding I did the more I learned you could do with epoxy. I decided it could replace shims on rifle bases if I did it right.
I played with bedding the 2-piece Leupold and Talley bases I was using at the time. It could be done but it was a lot of work. You needed scope alignment bars to determine which base needed to move and then you needed a long bar to bridge the two pieces in order to bed the pair. A 1-piece base made things much simpler. I was also of the opinion (still am) that a 1-piece base also adds some rigidity to your receiver.
Back then I would sell a 1-piece Leupold base if at all possible but as “tactical” guns, gear, shooting, etc. has become increasingly popular a few really good pieces of equipment have become available to civilian shooters. The piece that is most important in regard to rifle hardware is the one-piece M-1913 or Picatinny rail. This rail allows for industry standard dimensions and does not require rings to be twisted into dovetails or be lined up using opposing screws. Instead quality rings are machined as matched pairs. As a result you have what is potentially the system with the least induced stress.
To guarantee that stress is eliminated you bed it for the best platform for your rifle sight. If you just slop epoxy under the base and tighten it down you did about as much for your system as those stock bedding jobs I mentioned. What we are concerned with is where and how to bed a scope base.
The first thing I want to know is which end of my action is the end that needs fixed. Given that the barrel tenon is in your front receiver ring that is where I start. I place my base on the receiver and put the front screw in and tighten it down. That screw should secure your base to your receiver to the point where it will not wiggle. If the base wiggles it is too long and needs to be shortened so that it is not bottoming out before it secures the scope base; this important not just to the base but if that hole goes clear through the receiver (as it often does) you do not want to mash the threads on your barrel tenon.
Once I have ensured that screw is of proper length I will tighten down both screws on the front of the base. More often than not (unless your receiver and base are perfect mates) the rear bridge is where you will find you have a gap. Sometimes you can see daylight between the base and rear bridge. Other times you can just barely tell their is a gap but it is there. You can make that determination using paper or feeler gauges if you cannot see it.
If I can see no light or easily slip a feeler gauge under that base at the rear bridge (one of the the things they got right with the Remington 700 RR actions) I will install the rear screws and remove the front screws then reassess. I find the front has the gap far less often due to the way the rear bridges are ground and the fact that many actions these days are the same diameter front and rear.
Once I determine which end needs epoxy I plug the mounting holes with modeling clay. I do not use screws on that end because you do not want to compress the epoxy and the front will take care of the alignment.
I also plug the screw holes in the base. I do not want a mechanical lock above all else but I also do not like to clean up epoxy that has flowed into those counter-bored holes. An ounce of prevention is worth a pound of cure.
Use some sort of straight edge plane the clay flush with both the surface of the rear bridge and the scope base. Bolt down the other end of the base. Before you tighten the front end push the base all the way forward. If the base has a recoil lug (Seekins, Badger, Nightforce) the lug will engage the front receiver ring. If not the front mount screw holes will engage the screws. Either way, you get a positive stop that will stop the base from working forward (due to inertia) under recoil and will keep thing aligned on axis. With the base secured, tape off the receiver around the base to make clean-up easier.
Remove the base and coat the sides of the base and the metal of the receiver with a release agent (I like Brownells aerosol but any of the paste wax varieties work fine too). Mix up a two-part epoxy with high compression strength (Devcon, Steel-Bed, or even JB Weld) and spread a thin layer on the end of the base to be bedded. Install the screws on the opposite end of the base, press forward, hold, and tighten. Any excess epoxy will slowly flow out from the base while the rest settles into a stress free base. After a little while the excess epoxy will begin to firm up but will still be a little gummy. You can take a toothpick and trace the edge of the base and it will neatly cut away. Let the base harden for 24-hours before removing the base which will have two neat little reference marks for your mounting holes.
I drill those holes out with an undersized drill bit and clean them up with a tiny round file. I also give the edges of the epoxy along the bottom of the base a quick pass with a miniature file or fine sand paper just to ensure there is no proud epoxy showing (remember what I said about making things look finished). The modeling clay will largely peel out with the use of a toothpick and the rest easily swabs out of the holes with Q-Tips and pipe-cleaners soaked in mineral oil or just about any solvent. Give your action a wipe down with a solvent to remove the release agent then install your scope base to proper torque.