Shimming scope

[comuaiki]

I have a factory flattop AR with a Sightron Mil Dot scope, brand of rifle is not important. It has a one piece tactical scope base about three inches long. Here is the situation, the scope is adjustable out to 300 yards and can't be adjusted for more elevation gain beyond that. I would like to shim the scope and have read lots of suggestions on the internet that use the concept of using shim stock or a coke/beer can and placing it between the scope and ring on the bottom at the back end and on the upper on the front.

I am looking for suggestions or comments or your experience with shimming a scope to gain more elevation with my scope. I would like to get another 20MOA if possible and am not sure it torqueing a scope in this manner will eventually cause accuracy or instability with the scope.

Thoughts...

[DougGuy]

They make compensated bases for use on air rifles, one has 10 MOA built in, another has 17MOA yet another has 20MOA. Could you replace the base with a modified compensated base made to fit your AR?

Maybe copy how they compensate the airgun base and machine some off the bottom of your base to tilt it?

[Der Gebirgsjager]

My thoughts--scope shimming is always a bit tricky, but I don't believe that putting undue strain on the scope is a major concern unless something really radical is done. Many thin materials are suitable for shim stock. The coke can metal will work, as will layers of masking tape or card stock. Depending on your budget, some other alternatives are to play around with different scope mounts and ring combinations, finding one that is a bit higher in the front. Also, lower in the back will get the same results as higher in the front.

[Ballistics in Scotland]

You could do this with some rings but not with others. They would need to be split at 3 0'clock and 9 o'clock, or very nearly. With Conetrol invisibly and vertically split rings for example (a system which makes my toes curl up in ecstasy, although I have only one set bought in the 1970s), you couldn't.

20moa is about 20in. at 100 yards, about 0.2in. at one yard, and about 0.016in. at three inches. That would mean shimming the scope by 0.008 at the top of one ring, and .008in. at the bottom of the other. That can be done, and won't look to obtrusive, but don't forget that the shim ought to be about 0.00265in. thicker at one end of a 1/2in. thick scope ring than at the other end. Do the ring screws up tight and you may warp the tube or apply tension which will disturb the fit of the other shim.

What you could do is cut three successive bands of, say, 0.003in. shim material, one the full width of the ring, one a third that width, and the other two-thirds. use them to build up the effect of a tapered shim, and assemble the whole thing with black epoxy. I have an idea JB Weld make some, or you could colour it with lamp-black from eBay, or the black powder Brownells sell for the purpose - which may actually be a way of buying expensive lamp-black. Do it with the light screw tension with the scope barrel or a length of 1in. tubing greased but the rings degreased. Then disassemble and degrease, and you can tighten them as much as you like.

Another possibility with the horizontally split aluminium rings would be to temporarily glue a steel shim to the top of one ring and bottom of the other, attach them to the rifle or a spare scope rail in your vice, and ream them with a 1in. hand reamer. It will cut more into the aluminium than into the steel. Then remove it (quick epoxy or superglue was best, since high heat will remove the dye from black anodizing), and repeat the process with the shims fixed to your newly reamed surfaces. If you end up with rings that won't grip the scope, you can file metal from the surfaces where top and bottom halves of the rings meet.

There is a passage in "Zen and the Art of Motorcycle Maintenance" where someone's handlebars are turning in their mounts. His friend suggests that a piece of Coke can will work perfectly, but he gets in a state because he wants it done the proper way. Coke can is fine, though it may work best if you anneal it by heating nearly red hot to remove its springiness. There is never going to be any trouble making it fit a 1in. diameter circle, but you want flatten out the tiny curled edges made by scissors or modeling knife. I think ordinary masking tape is too spongy, but a good material is adhesive copper tape. It is common on eBay, as it is used to make Tiffany style lampshades, or for making barriers to stop snails climbing into your potted plants.

I hope you weren't looking for something simple and easy...

[Nobade]

Check out Badger Ordnance. They make an integrated AR type mount with various amounts of cant built in. Best way to go, no stress on your scope. Not cheap though.

-Nobade

[Ballistics in Scotland]

Very possibly I'm wrong, but I thought the OP meant a base in one piece with the receiver. If it screws on, and that Badger mount doesn't suit, epoxy-moulded shims under the base would be a better way to go than inside the rings.

I can't remember the brand of rings which use (or used quite a few years back) a plastic liner which actually met the metal rings on a surface which was an approximately 5/8in. wide section of a sphere. If it was possible to shim those, the the plastic sleeves would swivel to take up the angulat misalignment.

[Whiterabbit]

a good material is adhesive copper tape. It is common on eBay, as it is used to make Tiffany style lampshades, or for making barriers to stop snails climbing into your potted plants.

I hope you weren't looking for something simple and easy...

I have a massive roll of this stuff form a surplus store. I never thought about using it to shim a scope base. This is exactly what I am going to do.

Thanks for the suggestion.

[gunshot98]

I would shim the base if possible. If not, you can shim the rings, just don't overtighten the top caps.

[dddoo7]

I would not shim the rings if you are using a quality scope and rings. Most cheap rings are probably that much out of alignment...or will flex that much anyway, but with a high quality scope and good quality rings you will introduce stress.

Like was said earlier...there are plenty of companies that make one piece scope mounts for flat tops that have built in cant (like 20moa) and they will not stress anything.

I don't know how accurate you are trying to shoot...but stress in a receiver, barrel, or scope is never a good thing when it comes to accuracy.

[Buck Neck It]

Epoxy works good at first, but degrades from solvents quickly.

[Cap'n Morgan]

I can't remember the brand of rings which use (or used quite a few years back) a plastic liner which actually met the metal rings on a surface which was an approximately 5/8in. wide section of a sphere. If it was possible to shim those, the the plastic sleeves would swivel to take up the angulat misalignment.

SAKO and Tikka rifles uses the concept you mentions:

http://www.sako.fi/scope-mounts

I shamelessly stole the idea when mounting a Malcome scope on my 1885. Almost unlimited amount of free rotation and good clamping force:

http://castboolits.gunloads.com/showthread.php?93512-New-front-mount-for-a-Malcolm-scope

[Whiterabbit]

those rings are burris signature

http://swfa.com/images/420521.jpg

[Nobade]

Very possibly I'm wrong, but I thought the OP meant a base in one piece with the receiver. If it screws on, and that Badger mount doesn't suit, epoxy-moulded shims under the base would be a better way to go than inside the rings.

I can't remember the brand of rings which use (or used quite a few years back) a plastic liner which actually met the metal rings on a surface which was an approximately 5/8in. wide section of a sphere. If it was possible to shim those, the the plastic sleeves would swivel to take up the angulat misalignment.


It's an AR-15 flat top. Picatinny base milled into the receiver. The Badger setup is about the best way to do this, but the Burris rings mentioned would work as well just nowhere near as stout. Can't shim the present clamp-on base since it interfaces with the picatinny slots. He could shim the rear ring to jack up the scope, but that's a poor way to do it since it induces stress in the scope.

-Nobade

[fouronesix]

I'd think even if it has the universal Picatinny/Weaver type built-in base, the rear ring to base mate could be shimmed some amount for more elevation (within limits of course). Yes, that would offset the ring alignment and it would put offset stress on the scope or mar the scope. But, some lapping compound and ring lapping bar would easily correct that.

[Ballistics in Scotland]

those rings are burris signature

http://swfa.com/images/420521.jpg

Those are the ones, and I don't think they would be weak if they use the right kind of plastic, since it is covered by steel. If you put a shim on the top of one insert and bottom of the other, and at first only partially tighten up the rings while you mount the scope on the rifle, they should line up nicely.


The only thing is that if the insert to ring interface is a spherical curve, you would probably be better using some form of plastic shim. The copper foil or other metal shim would wrinkle.

[Ballistics in Scotland]

I'd think even if it has the universal Picatinny/Weaver type built-in base, the rear ring to base mate could be shimmed some amount for more elevation (within limits of course). Yes, that would offset the ring alignment and it would put offset stress on the scope or mar the scope. But, some lapping compound and ring lapping bar would easily correct that.

Brownells used to sell a double ended 1in. reamer, to turn with a wrench on flats in the middle. It is probably expensive, but although I have never done this, I think it would work nicely for the trick I mentioned with steel shims above and below, in aluminium rings.

[Whiterabbit]

that brand of ring comes with shimmed inserts so you don't have to shim them. Ever.

I don't know about not being able to shim the 1-piece rings originally referenced by the original poster though. I suspect that will be quite possible.

[fouronesix]

Brownells used to sell a double ended 1in. reamer, to turn with a wrench on flats in the middle. It is probably expensive, but although I have never done this, I think it would work nicely for the trick I mentioned with steel shims above and below, in aluminium rings.

Yes the double ended reamer is still available and makes the job very quick. The lapping bar with compound does the same thing but requires a little more time and effort.

Bottom line on this whole issue is if that top rail is halfway close to parallel to the bore, then I can't imagine most common modern scopes (including the Sightron) not having enough elevation to zero at pretty long distances beyond 300. Unless the top rail base is way out of bore alignment with the bore axis to receiver axis way out of kilter. Or the scope (with Mil dot reticle) already has a limited internal adjustment range where most of the elevation correction is based on the reticle rather than the internal adjustments.

Seems like shimming inside a ring is going to introduce a whole bunch of problems that aren't necessary. The Burris "eccentric" rings will probably work if that's the way the OP decides to go.

[Whiterabbit]

guys, the OP has a single piece tactical ring set. Shimming that will not misalign the rings.

http://media.midwayusa.com/productimages/880x660/Primary/386/386787.jpg

[fouronesix]

OK, that's two different versions of what the issue really is. A milled-in universal rail with separate rings vs a one piece base ring unit. If it is a one piece unit then it's an obvious no brainer. Shimming the rear end a little (between the unit and the rail) should solve the problem. It would be interesting to find out exactly how much elevation is needed since the current scope set up is maxing out at 300 yards- something just doesn't seem right about that.

[Whiterabbit]

There are not two versions of the real issue. We brought up the two ring issue, not the OP. OP is using a one piece unit. 2pc ring discussion is not directly helpful to the OP's issue unless he cites parts replacement as an option to solve his problem. He did not cite that as an option.

If it is a high-magnification scope, they don't have a lot of "up". Also I have no idea how well his 1 piece tactical scope base is made. For all I know, it's an airsoft base from a gun show. Brand of upper is not important, so who knows how good that is. He also doesn't say cartridge, for all I know he is trying ot shoot subsonic 300 blk to 500 yards and runs out of adjustment at 300. I just don't know. And I don't think we need to know to give him a good answer to his question.

That the ideal shim solution, if he wants to solve his problem by shimming, may be to place shim material on the picatinny rail under the rear attachment screw of his 1-piece rings. I REALLY like the Scot's suggestion to use copper tape, that would be really ideal in this case, since the OP could cut strips and it is self adhesive. I'll be trying that under my 1-pc rail on my MarkX for sure.

If you want to ask all sorts of questions about rifle brand, 1 pc base brand, cartridge, shooting goals, etc, knock yourself out. Maybe you will find that shimming the base is putting a Band-Aid on the real problem. Maybe not. But the OP gets the final say on what corrective action is practical for him. If he felt that he did not have a clear understanding of how what he was doing and equipment choice affected his shooting goals, he would have specified this up front (including cartridge, shooting goals, brands, etc). So I interpret his question not as "solve my problem for me" but rather "tell me about shimming methods and whether my described shimming method is valid or not". which we of course suggesting is not (valid, shimming the ring). That's all.

[fouronesix]

Here's the OP's description. "It has a one piece tactical scope base about three inches long."

Well to me and likely many others that leaves a lot of room for interpretation. So it was not as clear as you would have all believe… given the large number of variations of scope mounting systems for ARs and the countless varieties and designs of ARs.

I think the OP has several options to get the added elevation. But the game of "20 questions" with the sure-to-follow "20 guesses" as it was done is this thread probably isn't the best.

[Ballistics in Scotland]

that brand of ring comes with shimmed inserts so you don't have to shim them. Ever.

I don't know about not being able to shim the 1-piece rings originally referenced by the original poster though. I suspect that will be quite possible.

Ah I didn't know that about the Burris rings? You mean inserts thicker at the top than the bottom, so that you can turn one upside down? If not, it would be possible to turn them on the lathe. I think if you drilled a pieced of steel to the radius of the insert's edge, and use that to make a lathe took, you could turn aluminium or a hard plastic in one cut. The 1in. hole could easily be made the necessary amount off-center.

[Ballistics in Scotland]

guys, the OP has a single piece tactical ring set. Shimming that will not misalign the rings.

http://media.midwayusa.com/productimages/880x660/Primary/386/386787.jpg



If, on the other hand it is what you illustrate, rings integral with a one-piece mount to clamp onto the Piccatinny (née Weaver) rail, a full-length shim under the mount would be the way to do it. I would temporarily soft solder a piece of thin brass sheet to a metal block, and belt-sand it until it is about .01in. thick at one end and .02in. at the other. I think that mount would clamp the scope down firmly on that. When it is done and works, you could epoxy that shim permanently to the mount.

Either epoxying to the rail on the rifle or milling a slight slope on the top of that rail would probably work as well, but would deface a bit of the rifle you want to keep pristine. Nobody sits and admires the underside of a scope mount.

[brstevns]

Depending on how much shimming is needed I have used the Aluminum tape to shim a few. The same type of tape used to Beagle a bullet mold.

[Whiterabbit]

Ah I didn't know that about the Burris rings? You mean inserts thicker at the top than the bottom, so that you can turn one upside down? If not, it would be possible to turn them on the lathe. I think if you drilled a pieced of steel to the radius of the insert's edge, and use that to make a lathe took, you could turn aluminium or a hard plastic in one cut. The 1in. hole could easily be made the necessary amount off-center.

That's exactly right. They make several inserts:

-concentric
-10 MOA (estimated) off concentricity, thick clamshell at the top of the ring and thin at the bottom (and vice versa for the other ring)
-20 MOA, see above

etc. Or used for dialing windage for center if you want, etc.

[country gent]

The shown base cant really be shimmed underth as it throws the side s out of alighnment. I have made add on rails in 10 moa 15 moa and 20 moa with it built into the add on rail these also had a side mount for redfield or warner rear match sights. Raising the back of that mount will have only the bottom surface of the clamp and fixed side bearing the top surface floating. The mount could be set up in a mill at 20 mins on a sine bar to a fitted rod in the rings. Remil the base and sides to match would be very touchy work and require getting intimate with gage blocks and a indicator. A shim and epoxy ( Plastic steel putty, JBweld, or a thick bedding compund. Could be used. Coat the shim with release agent, coat the rifles rail very well with release agent. Bedd just the bottom flat of scope mount to start and let cure. remove and clean excess out. recoat the rifles rail with release agent and coat the clamps vee and fixed vee apply epoxy and snugly clamp in place. ( This gets the side vees and clamp back in the same plane as the new base. Should be very solid if done correctly. One thing to remeber with a 20 min base is it may not zero at 100 yds. My one long range rifle scope is at mechanical center or very close to it at 900 yds it barley zeros at 200 yds with this base.

[fouronesix]

If in fact the OP's unit is similar (or same) to one shown in picture, it is common for some of them to already have about 20 minutes of positive elevation built in. As long as we are in "20 guess" mode, any possibility the OP's unit is one of those with the elevation angle and is mounted in reverse, thereby most of the scope's elevation is being used up in correcting for the negative elevation??...with the scope's adjustment being maxed out at about 300 yards.

The scope itself should have at least 20-30 minutes of elevation built in. If the receiver and bore line are even close to parallel, that should be plenty to get to well beyond 300 yards! So something else may be going on. But, I repeat myself.

[fast ronnie]

Ah I didn't know that about the Burris rings? You mean inserts thicker at the top than the bottom, so that you can turn one upside down? If not, it would be possible to turn them on the lathe. I think if you drilled a pieced of steel to the radius of the insert's edge, and use that to make a lathe took, you could turn aluminium or a hard plastic in one cut. The 1in. hole could easily be made the necessary amount off-center.

Burris signature rings have an insert that can be ordered concentric, .005, .010, .015 and .020 offset. I just used a set of these to correct an action with the screw holes .025 out of line with the barrel by turning them sideways instead of up and down. They are available thru Brownell's. They can either fit a picatiny rail or standard leupold type cam bases. They are also available in both 1" and 30 mm.

[comuaiki]

Wow, lots of great ideas with lots to think about. At the time of the posting I did not think this would be such a complex issue with so many variables and how shimming a scope could physically affect it.

Here are a few more things about my setup which could help clarify things a bit. Since the AR is a flattop the picatinny base is definately milled into the reciever, the OP rings are 30mm and come with a plastic sleeve insert to hold my one inch Sightron scope tube and is simmilar to the one posted in this thread. I have to admit I never did lap the 30mm rings first and am now not sure if I should lap the one inch inserts or how to do that. If after lapping I still have the problem then I will probably head down to the local Auto Zone and buy a two dollar stainless steel feeler gauge close to 0.016", cut it and shim the rear of the OP.

Ontop of that the scope is mounted a bit low for my taste and have picked up a TAPCO INTRAFUSE riser to gain a bit more height for the scope. Do you think by introducing a riser into this equation it may cause additional problems? Whould it be better to scrap the riser and currennt OP and buy a different OP which is higher than my current setup.

This whole AR thing is new to me and is my first and only to date, however I have been contemplating onn purchasing a 6.5 Grendel or a 264 LBC, but that is another story with a whole different set of quality equipment.

[Silverboolit]

Have you tried to level the barrel and then check the rail to see if they are both leveled (parallel ) with each other? I do this with my airguns to check for 'barrel droop' and have done it to many powder burners too. It helps to know where you can start with shimming the scope or the bases. I lap all of my rings and you will be surprised at how out of round they are from the manufacturer. Pyramid Air should be able to help you with the compensated mounts/rings. Check their website for what's available.

[country gent]

The risers can be a big help if scope is low. But you are introducing another item into eqution also. I made my own risers with 10-20 mins built into them and a mount for an appeture sight on the side. The addition of elevation in the base helps keep the scope in center of optics at longer ranges, and also affects it zeroing at 100yds and 200yds. My AR was an NRA High Power match rifle in 223 and was routinely shot out to 600 yds in matches. Add the rail and watch windage when zeroing. Elevation shouldnt be affected also with a straight rail if it has any built in itn the rail thats diffrent. I have lapped rings for years to insure proper alighnment and that the scope isnt being stressed or bent in the rings. With a round bar the same dia as the scope. mount rings on rail. remove tops to start. Coat bar with a med fine lapping comould and lay in rings gently. now work back and forth in a figure 8 pattern rotating on axis while sliding back and forth. Watch the rings when you see 40%-60% clean up install tops lightly and work till bottom halves ate 80%-90% cleaned up. Rings are now in the same plane and perfectly alighned. On good rings this dosnt take but 15-20 mins to do. Some rings take longer as they are out more. Lapping rings is another tool in or box to help get the upmost accuraccy and or consistency.