Home Forums Hunting & Shooting Ask The Gunwriters Shimming a Scope

I took a new (to me) custom Springfield 1922 in .22 Hornet out to shoot yesterday. The rounds were landing approximately 7 inches high at 50 yards and there is no more downward adjustment left on the scope. Thus it appears that I'm going to have to shim the scope.

I'm assuming that I will need to add height to the front ring or under the front base (the bases are mounted on the receiver ring). Approximately how thick a shim would I need to insert and would I be better off placing it in the ring (and if so, how do I deal with the reduced interior ring diameter) or under the base?

Thanks.

I would shim under the base, try a .004 shim.

Soda pop or beer can material makes grate shim.

Yep, cans work great, and can be easily clipped to the right size.

Have also had good luck shimming the inside of the front ring with duct tape. You want to just place it on the bottom, not on the entire semi-circle. If one layer isn't enough, an even smaller piece can be added in the middle of the first one.

Shimming is also a good way to get ring marks, because if one ring is higher than the other the scope rides on the edges.

I do a version of what Mule Deer recommends with duct tape. I just put a piece of tape in the bottom, but not all the way around. Then I test fire. If all is good, I put epoxy in the ring bottoms and use a precision steel bar instead of the scope, release agent as appropriate. After the epoxy sets, I peel the tape out and fill the remaining gap with epoxy, placing the bar as before. If done carefully, you end up with perfect stress free scope contact in the rings. No metal removal is involved, and the epoxy can always be removed with a good solvent down the road, if desired.

This is what I do if Burris Signature rings with inserts aren't applicable. Works well for me.

Paul

Just went through the whole 'shimming' experience.

To 'lower' your POI, shim UNDER the scope on the front ring.

A soda bottle or other adult beverage aluminum can will give you a shim that will change POI about 4". Cut about 1/2-1" long strip, narrow enough to 'hide' under your scope. I'd think 2 shims, maybe 3 will get you where you need to be. Probably just 2.

I've used electrical tape on the lower ring-half, and it worked great.

That's if the rings were aligned and you shim them out of alignment. It seems that if he runs out of vertical adjustment then the rings are out of alignment already, and shimming would be bringing them in.

That's one way to look at it, but I don't know a precise way to gauge whether rings are in alignment to begin with. I don't trust rods, although perhaps they work if used correctly. What I left out is lapping as a first step. If you start lapping you see pretty quickly where the contact is, and it can be an eye opener. If they are out of alignment it will show as wear (bluing removal) on the respective edges. I have lapped a lot of rings, and can't recall any that started out in perfect alignment or contact. Maybe "good enough", but far from perfect.

If I can't use the Signature rings I usually routinely lap and epoxy bed, even if shimming isn't needed. Just gives me more confidence.

Paul

What type of bases are on the receiver? If they are anything "standard" I would seriously look at the Burris Signature rings with the plastic offset inserts. They do a wonderful job of correcting alignment issue of scopes, both vertically and horizontally (or a combination of both!). You can find a combination of plastic offsets that will allow you to keep the scope turret adjustments roughly centered so they aren't running near or at the limit of the internal springs. Plus they don't mark up the scope body and slippage won't be an issue on a .22 Hornet. Just my $0.02...

When I have this sort of misalignment, I generally use five-minute epoxy to bed the scope into the rings. If I need to raise the front ring, (and, in this case, it hasd to come up quite a bit) I cut a thin strip of business card which will lie in the front of the front ring. I then wax the scope tube, degrease the bottom half of the rings, mix the epoxy and put some on each ring. Then, put the scope in the rings (don't dawdle!) and snug the tops down (not too tight. just enough to bottom at the rear of the rear ring while the front is supported by the card). Let it set up for a half hour or so then remove the scope and trim off the excess epoxy and there you have it. The bottom halves of the rings will be perfectly aligned with the scope tube and the scope will be sitting in the correct attitude. GD

I've used soda can pieces wrapped in duct tape inside the rings when my $90 supposed to be high enough according to the web rings weren't high enough. Too frustrated.to send back and start over so just shimmer them. So far so good.

What Greydog describes is essentially the same as my method, but a bit simplified. I don't like the idea of bedding the scope directly into the epoxy. I worry about having it stick and needing to knock it a bit to get it free, so I use a length of precision ground 1" steel rod to form the epoxy bed.

Even that may not be a perfect fit. As Mule Deer noted elsewhere, scope diameters are not always true to spec. Folks, when you consider the various components made by different manufacturers, it's taking a lot on faith to expect everything to fit perfectly. There's this thing called stacking tolerances. Sometimes you get lucky, sometimes you don't.

Paul

A little something I've been playing with.



An old Balvar 8 with really clear glass. I use windage adjustable bases or rings and shim for elevation. I call it the "Turret Twisters Nightmare", you'd be surprised at the number of baffled shooters at the range when they notice there are no adjustment knobs on the scope. I've since moved it to a Savage 22-250 with Leupold windage adjustable base, shown are Weaver windage adjustable rings. I use greased hard paper shims(computer paper) they don't continue to compress over time and are easy to insert by just loosening the ring screws.

yes shimming works but i really hate to shim,i would rather get the right mounts for my rifles if i plan on keeping that scope on that rifle for a long time. scope mounts are not that expensive.

I've never owned the Weaver windage adjustable rings. When I look at them I conclude the design is flawed. It seems to me that if a set were mounted on a one-piece base (or the top of a flat top AR type rifle, or a two-piece base well anchored so as to act as a one-piece base), adjustment would create two axis that were parallel, but not collinear. If so, this would torque the scope tube, and possibly lead to ring marks.

However, I should make it clear that I never purchased a set of rings and tested them to see if I am correct. I have the education and experience to believe I am correct, but I will not state it as fact unless I am more sure.

Redfield style windage adjustable rings are supposed to allow the front ring to rotate to "point" to the rear ring. The rear ring should theoretically not turn to "point" toward the front ring, but it appears to me that there is enough "flex" in the design to minimize the issue. I don't see much "flex" in the Weaver design to allow the rings to align. (I don't like the Redfield system, but mostly for other reasons.)

All that said, I really do like the "plain old Weaver" rings and bases, especially the steel bases. But I remain skeptical of the Weaver windage adjustable rings.

Another bad point about the Redfield windage adjustable system is the adjustment screws have less purchase on the bottom of the ring when they are adjusted away from center. The adjustment screws move on a line left-right, but the rear ring moves in an arc.

Absolutely. That is what I was alluding to above. If the rear ring turns as it move laterally (following the arc you mention) to continue to point toward the front ring, then either there is enough slop in the threads or flexibility in the windage screws to seat correctly with the bottom of the ring . . . or not.

If things are not designed to line up correctly (Burris Signature system, rings reamed and/or lapped, scope epoxy bedded in the rings, whatever the method that ensures alignment), then the mount is going to try to bend or kink the scope tube. Depending on how stiff and strong the scope tube is compared to how much "leverage" the particular mounting system has, this can be bad or not so bad. But however it works out, it is not optimum. It has always seemed to me that even if the scope is not overly compromised, the whole deal is not a good candidate for holding zero.

Been gritting my teeth but can hold it no longer. If a scope won't zero something is wrong between the scope and the rifle (assuming the scope is good). If you shim it there is still something wrong between the scope and the rifle. Petty little things like that wear on me. Silly I know but I feel much better with things done "right."

Yes it does. Cuts easily with a scissor too.

Of course something is wrong, if not there would be no need to shim

FWIW, it's a custom rifle with mounts and rings I can't identify and, given my luck in these things, oddball spacing in the base mount screws. I'm looking for a simple solution versus having to fill and then re-drill and tap screw holes for another type of base. Sometimes, the perfect is the enemy of the good and in this case I'll be satisfied with the good.

In that case you can go to Brownells (for one) and buy steel shim stock and cut a shim to match the scope base. The idea is to blue shim and have no unsightly gaps or lines between the base and the receiver. Brownells also sells a scope shim kit punched out of steel and blued. At least try to use one decent looking shim rather than a stack of pop can bits.

scope shim kit

steel shim stock assortment

just put a custom picatinny base on this rifle should take care of your problems, i put custom picatinny bases with 10 degree on my number 1`s before anyone made them.

[quote=erich]A little something I've been playing with.



An old Balvar 8 with really clear glass. I use windage adjustable bases or rings and shim for elevation. I call it the "Turret Twisters Nightmare", you'd be surprised at the number of baffled shooters at the range when they notice there are no adjustment knobs on the scope. I've since moved it to a Savage 22-250 with Leupold windage adjustable base, shown are Weaver windage adjustable rings. I use greased hard paper shims(computer paper) they don't continue to compress over time and are easy to insert by just loosening the ring screws.[/quote


And I thought I was the only one to do this....I love those tapers cross hairs...I have one mounted to a rem target master I rebarreled to 17 hm2. Cool setup...

I had a Savage 99 that ran out of windage. I bought the Burris offset inserts. They fixed the problem. They will correct your elevation problem also

Another option is to lap the bases. Sounds like the rear one needs to be lower. Depending on how much height difference you need I would remove the bases and wrap the action with 220 or finer grit wet or dry sand paper and stroke the base over the mounting area lightly. Look for full contact and remount to check alignment frequently. This could reduce the need for shims or possibly make them unnecessary. Depending on the type of mount you could also remove material from the top of the base but harder for this not to show although cold blueing will make it not so apparent.

There is a formula for calculating how much the mounting is off but I would have to look it up, it is for iron sights but may adapt to scopes. Or you could shim till the scope is centered and use that as the measurement. But if it takes more than some tape or a single shim I would turn it over to a good gunsmith.

Paul[/quote]


I use this method with good results as well. I figure that If the scope tube is not straight, at least I’m not stressing it. On the other hand, if I was switching scopes on a rifle or planned to, I would use the precision steel bar method. These are both sound ways of going about this.

Every scope I put on a rifle is glass bedded to the rings with none other than JB weld.

I would check to see if there is shim under the rear base before shimming the front base or ring.

My dad always taught me, as a rule of thumb .001" worth of shim is ABOUT 1" at 100yds. If your 7" high, you will want atleast .007" of Shim. I believe Pop/Beer can material is appx .010-.012" thick

Just do a similar triangles comparison if you want a better approximation.

You need to be careful with the lapping. Even one thickness of sandpaper increases the radius of curvature of the action. If you fully lap to that radius, when you remove the paper, the base now has a larger radius of curvature than the action. The base will make contact with the action only on a line between the screw holes. Then you will need to epoxy bed the base to the receiver.

The formula for scope shimming is the same as for sights, but you use the distance between the rings instead of the distance between the front and rear sight. As mathman mentioned, similar triangle is the way to think about it.

Some very good ideas here, but for the love of easy, shim the durn base! Easy.

Guys used to do this for long range shooting ALL the time before 20 and 30 MOA scope bases were available. You might need longer screws, or you might not. This is not just a good solution, it's a great solution. Don't honk the scope rings down super tight yet (which in theory might mark the scope tube, shim the base until you're somewhere in the middle of the scopes adjustment range, then, if you want to be anal, lap the rings (with the scope bases at their new height).

I have done this many times, used beer cans, Devcon, beer cans plus devcon, etc

Burris Signature Rings with offset inserts. Works great to align scope, holds scope well with hard kickers, won't mar scope either. Return scope to optical center by adjusting cross hairs looking thru scope up against a mirror and "line-bore" looking thru bore and you should be good to go. If you use dual dove tail mounts, the Burris Rings will work with Leupold bases which have a better looking sculptured appearance to me. This is of course if Burris still offers this product. Have used them many times in past with very good results. By using a .030" insert in front and back, you can get up to .060" offset if needed. Several insert sizes to choose from. Oh yeah, I'm not a Gunwriter. Just have run into this more times than I should have.

Aw gee, don't make it complicated.

If there's not a lot of shimming to do, and it's not a loudenboomer, a pop can and scissors are your friend.

1. To get an idea of HOW MUCH shim, measure the distance between rings, get out your calculator, divide 100 yards (3600 inches) by your rings. You want seven inches (thats 14 inches at 100) PLUS whatever it takes to get centered, so you should spin the turret to see what your total adjustment is. Half of THAT gets you what you need to hit optical center, which is what you want.

2. You need to decide whether to shim INSIDE the ring, or UNDER the front base. It might be best to get a couple of slightly-longer base screws and shim underneath, because that doesn't mess with the inside diameter. If your total rise is more than one turn of the screw and you have less than three turns, you definitely need to get some longer screws to use all the thread you can.

3. In all cases, it's good to at least CHECK ring alignment and whether or not lapping is needed. If the alignment is bad, I'd shim the bases with longer screws, then lap it good. If alignment is pretty good and you don't need a mess of shims, just plopping a shim about 1/2 wide on the centerline should work okay just as long as you don't crank the ring screws too hard.

I have done:

1. Shim rings, with a skin-coat of JB Weld as an adhesive, then lapped that. You'll need some kind of release agent on either the scope, lapping bar, or whatever straight round stock you use. I also ruff up the inside of the lower ring to give some "grab,"
2. Shim base, again using JB as a gap filler. Lap rings after. If you put JB or some other good epoxy on both rings, you might not need to lap at all. There's also some Loctite green stuff for bedding, Mic McPherson wrote about that. Works for bases, I don't think he's used it on rings, and by the way, he's a huge fan of the Burris setup.