shims under scope bases

[Rob]

The other day, I mentioned to a young fellow that I was shimming a scope base and he told me that there must be something wrong with the rifle.

A lot of guys who only have experience with relatively new (i.e. CNC machined) production don't realize how much hand work was formerly used in finishing firearms. The machining tolerances were just not all that close to spec, so hand finishing was required...therefore there is a bit of variety in the dimensions of individual rifles. (The reason for the "matching numbers" fanaticism on the part of milsurp collectors.) I want my two-piece scope bases to be actually aligned, not just close enough. (With the once more popular one piece scope bases, stress caused by slight misalignment could easily go unnoticed.) With a lot of older 20th c. rifles it is necessary to shim scope bases to achieve a level fit and to take the stress off your scope.This doesn't just apply to old Mausers. There are all sorts of quality rifle brands that can sometimes require shims to get the supposedly correct bases to align properly. Don't be afraid to shim...they can be purchased or easily made from shim stock available at any good hardware store, or from pop cans...I like steel, but some prefer aluminum or brass...doesn't matter much.

 

[stewbud]

I have a Ruger 10/22 with a droopy barrel and have resisted a shim and screwed around endlessly with offset mounts and special scope rings with offset inserts - with less than stellar results.

I shimmed a month ago and could not be happier with the result.

I got my brass shims from Lee Valley. A lifetimes supply in 6 thickness' in a box was in stock at my local store.

 

[rosconow]

I will/ have used an aluminium can cut into a sized strip for a shim when needed. Works good, a lil epoxy if needed as well.... viola!

 

[guntech]

I have on occasion glass bedded two piece bases on uneven actions... I would have the rings installed and a solid 1 inch bar clamped in the rings... loose screws in the bases ...when the epoxy cured this assured the scope would not be stressed at all. Release agent on the action, clean bases for the epoxy to adhere to.

 

[stampede]

I have on occasion glass bedded two piece bases on uneven actions... I would have the rings installed and a solid 1 inch bar clamped in the rings... loose screws in the bases ...when the epoxy cured this assured the scope would not be stressed at all. Release agent on the action, clean bases for the epoxy to adhere to.

Good advice. Thanks

 

[Power Pill]

doesn't lapping the scope rings fix alignment problems?

 

[guntech]

doesn't lapping the scope rings fix alignment problems?

It helps for sure... but the better the alignment is to start with, the less you need to lap...

 

[TungstenJohn]

I bought a JM Marlin 1895 around 10 years ago and it actually came with plastic shims and a note saying that I might need them - and yes I needed them so that I could actually raise the scope enough to hit the target- Marlin obviously realized they had made a boo boo in their machining - sold the gun anyway

 

[bearhunter]

doesn't lapping the scope rings fix alignment problems?

Sometimes you have to lap so much it ruins the fit between the rings and the scope tube.

Guntech has likely seen more than one decent set of rings ruined by aggressive lapping.

Back in the day, when different "customizers" were modifying milsurps into more fashionable sporting rifles, there weren't any set standards. Often, they did proprietary mods, so they could sell proprietary mounts. Sadly, they sometimes weren't able to supply the mounts and people had to improvise.

As such, it wasn't uncommon for the mount holes to be improperly spaced or askew from the axis of the bore.

Same thing happened with scope tubes. They varied slightly in diameters. Not much but enough to cause issues.

Then, there were the bases and scope rings themselves that had generous specs, to be polite.

There were a few companies, such as Tasco and Weaver, that you could almost set a micrometer by.

Today, we're spoiled. The latest tech provides us with tolerances that are extremely consistent and trustworthy. There is the odd time when offshore products don't quite match to the North American made products they're intended or claimed to. That's when lapping comes in.

I just mounted a scope on a very nice, hardly used Rem 721 long action, chambered in 30-06. The "new" owner of this very well kept rifle, told me he "got the rifle really cheap" because it wouldn't shoot well.

It had a period 4X Weaver Steelite scope that looked as new as the rifle. Parallax was normal for such a vintage scope, everything was tight and it all looked good.

I bore sighted it and it was quickly obvious that there wasn't enough elevation adjustment to bring the point of impact up to where the crosshairs indicated and almost all of the windage had been used to bring the crosshairs left.

Upon closer inspection, the rear base was a factory Weaver base, but it was askew and to high. The front base was also askew, in exactly the same direction.

I hoped against hope that the bases were the problem. No such luck. I have no idea how the Remington factory managed to screw up those holes so accurately or badly.

Whoever installed the bases and rings, had tried to "LAP" the rings for a better fit. The screwups just kept going on, until the rifle had been regulated to the back of a closet and sold for $100 to an unsuspecting youngster. The seller was at least honest enough to admit he had never shot his "Dad's" rifle, because he didn't like guns and he also knew it wouldn't hit anything it was pointed at. There's more but it's not worth it.

I looked through my bins and found an old steel, one piece base for the 721 long action, made by Bausch and Lomb. I filed/countersank the front hole, of the rear section, so that I could swing the front of the base over the rear receiver hole, which was actually on the axis of the bore, just like the rear hole.

Then the scope was again mounted in another set of rings, temporarily to see if it could be bore sighted. Thankfully it was OK.

The scope base had a shim coat of Acraglas applied and was screwed down tight to set in place. Tacky, I know but quick, cheap and will last forever.

The offset of the bases, caused the rings to "dent" the tube in two places. Parallax was horrific at best.

Went to the used scope shelf and found a similar period 4X Redfield, which was an upgrade from a not to bad for its time Weaver.

The youngster that owns this rifle will never be a hand loader. He's not the type and even though I've offered to teach him, can't be bothered.

So he goes to the LGS and purchases the cheapest ammo he can find, that's legal to hunt with. That 721 really shoots well. It shoots everything of the same weight into 2 moa without a hiccup.

The youngster gave me a sideways look, when I told him, "it's going to cost you $200." That's what the parts I put on cost me. No charge for the labor.

He told me that's more than he paid for the whole assembly. I told him I would give him $150 and keep the rifle.

We're not talking about a 16 year old here. He's 25+. He went to the bank and brought back $200.

Yes, it was a long story.

The thing is, lapping is only for minor offsets, to take the pressure off the scope tube, which effects parallax.