TargetTalk

A while ago, I decided to mount a dot sight on my Ruger Mark II pistol.

I attached a Weaver base to the receiver and mounted an UltraDot to it using the vertically-split, aluminum rings that came with the sight.

The sight has a round front tube and a round rear tube (each 1" in diameter) that are connected to a square-shaped central housing.

Before tightening the ring screws, I pushed the sight forward so that the central housing was contacting the front ring. I read somewhere that by doing this the central housing acts as a recoil lug, thus preventing sight movement.

Looking at my pistol now after quite a bit of shooting, I can see that the
Ultradot has moved REARWARD within the rings by about an 1/8"! The pistol's zero has not changed however.

How can this be? I have the ring screws tightened down as much as I feel comfortable doing; any more torque on them and I might crush the sight's tubes.

Even if the ring screws were too loose, wouldn't the sight tend to move FORWARD within the rings according to the Laws of Physics?

Do I have defective rings? Is this common with vertically-split rings? Any comments or solutions would be appreciated.

The vertically split rings are not your best option.
Basicallly you have a slick surface, the scope, contacting another slick surface, the ring, and you are right, movement is possible, unless you over tighten the rings. You can snug the rings pretty tight as long as neither of them are around the spot where the glass in the scope is. However screws do work loose from vibration, and a little non permenent loc tite on the screws is a pretty good idea, as soon as you are pretty sure, the scope is where you want it.
I suggest that you get horizontally split rings, and try those. They hold zero better, as the lower rings act as a cradle to hold the scope.
If you still have issues with the scope moving in the rings, you might want to try a couple of strips of two sided sticky tape around the scope at the points where the scope contacts the rings.

Dont use to much loctite and be sure to clean the screws and rings....As the previous post mentioned line the inside of your rings with tape . I used the blue painter tape ...My ultra dot doesnt move and its on my .45 1911...

Loctite 222 is the low strength type - it's good stuff for gun screws in general and you don't tear up the heads getting them loose when you need to.

Roger

Sometimes the inside of the rings are quite slick. Rough them up with sandpaper prior to installing the sight.

All good tips! Use thin double-sided sticky tape.
The un-intuitive motion of the scope in the rings depends on the energy of the cartridge, the recoil spring, tightness of the slide, etc. All those accumulated forces will decide whether the net result is scope slip forward, or backwards.

Thank you all for your advice; some good solutions for my problem.

The reason I chose to use the vertically-split rings that came with my sight? They are only 0.600" wide. Being this narrow, it is easy to avoid placing them on the sight tubes where a glass lens may also be located on the inside.

I have seen several horizontally-split ring sets, but they all appear to be much wider in dimension (over 3/4" or 0.750"). Using these, I am afraid of clamping over 1 of the lenses. Are my concerns here unfounded?

Can anyone suggest a specific brand and model of a horizontal ring set that works well with an UltraDot? I have used some of these types when mounting traditional scopes on rifles, but have not tried any with a dot sight.

I will admit it is easier to work with the horizontal types than the verticals!

There are many good 1 inch rings. I personally like the Burris Zee rings, but I have a 30mm red dot. I would not worry too much about the width of the rings. With the horizontally split rings, gravity does a bit of your work rather than squeezing the scope from the sides. Also a bigger ring will spread the force over a larger area and you will be less likely to crack the glass in the scope. I have never cracked the glass in a scope, and I have had rings on several different guns.
If you want more weight on your gun, use steel rings. If you want less, get aluminum rings.

You can always try the beer can shim (shooting and the zen of pistol maintenance).

A fellow shooter in my league had the same problem as you experienced; his UltraDot also moved rearward within the rings after awhile.

He was using the same vertically-split rings as well and felt the ring screws were sufficiently tightened.

He came up with a really cheap fix.

He repositioned his sight forward again until it came to rest up against the front ring. The gap between the central housing and the REAR ring now measured out at a tad over 1/8".

From a local hardware store, he bought a flat, black rubber gasket in the plumbing department. It had a 1 1/4" outside diameter, an inside diameter of 1" and was 1/8" thick.

Stretching it a bit between his fingers to make it easier to work with, he simply placed the gasket between the central housing and the REAR ring.

There was still a slight gap of a few thousandths of an inch now present between the central housing and the ring.

After shooting it awhile, the sight did move rearward again, but was solidly stopped after seating itself against the gasket. The pistol's "zero" has never changed throughout this whole ordeal and has been maintained ever since.