Air Rifle Scope shooting low

[shahid]

The new Nikko Stirling scope for the Diana Model 35, .22 Airrifle arrived yesterday with Nikko Stirling single mount.

It is a 4-12 x 42 mm. Quite adequate.

However when tested at the range with the elevation screw set to maximum up it is still shooting a good six inches low at 10 m on the range.

Any remedies ? Tried putting some ALuminium foil in the rings to raise the scope a bit but didn't work. Thinking of experimenting with higher ring mounts.

[Mack The Knife]

Sounds like a barrel droop problem.

Try the following:

1) One photo negative cut to size and placed in the bottom of the front mount.

2) Buy a RWS or Sportsmatch adjustable mount. Do your rough zeroing using the adjustment screws and only use the scope's adjustment for fine tuning.

Remember to bring the scopes vertical adjustment to centre before trying either options.

Mack The Knife

[shahid]

Thanks Mack The Knife. Will go for adjustable mounts now. The Nikko Stirling mount does have elevation adjustment but even that didn't work.

[Mack The Knife]

Hang on a minute!

I did not know Nikko were giving away adjustable mounts with their Gold Crown Airking series. I assume you bought a scope from that series. I always thought they were the plain Jane variety.

Anyway, if a Nikko branded mount (they don't make them and neither does RWS) does not work, then there is no certainty that the RWS mount will.

In your preceeding post you said, "tried putting some aluminium foil in the rings to raise the scope". This implies that you tried to raise both rings. If so, would I be right to assume that you raised BOTH rings of the adjustable mount? If you have done this, then this will not raise the point of impact. In this particular case you should raise the front ring and lower the rear, so that you are centered at your desired zero range.

Please try this, if you haven't done so already. I'd rather you adjust the mount than shim the rings as the latter can crimp the scope tube.

Mack The Knife

P.S.: Come to think of it, creating too much of an angle between front and rear rings can also crimp the scope tube.

[shahid]

No, the adjustable mounts were not free. Paid an extra GBP 11 for them, assuming they would be better. It is marketed by them, not sure who made them. The box is Nikko Stirling.

The Scope is Nikko Stirling - Gold Crown AO, 4-12x 42 . This was purchased for 54 GBP. All weather see through covers and rubber seals on turret rings and parallax correction rings come standard with this msel, the lenses are coated for better clarity and twilight factor increse.

It should be a great scope except for this shooting low episode.

I had raised only the front ring. Let's see the high bridge ( see through ) mounts. I am optimistic on this experiment.

Meanwhile the iron sights of the Diana 35 have been zeroed in. It is shooting perfectly, 1/2 inch groups at 20 yards centre to centre with Gamo - Hunter dome shaped field use pellets.

[Mack The Knife]

I had raised only the front ring.

Hmmm...

Could you do two things:

1) Look down the bore to check if the barrel is ever so slightly bent.

2) Check this scopes tracking accuracy (horizontal and veryical) on another rifle.

This scope is a relatively old model but has always been well regarded but do check it on another rifle or with different mounts to get that doubt out of the way.

What do those Gamo pellets weigh?

Mack The Knife

[shahid]

Gasmo hunter - .22 Cal 17.4 grains. Dome shaped. Box of 200 Dhs. 10 or Indian Rupees 125. Gifts are carried to India.

[Mack The Knife]

Try a 14.5 grain (max) pellet such as a RWS Superdome or H&N Field Target Trophy, if you have them with you.

Mack The Knife

[Mack The Knife]

Meanwhile the iron sights of the Diana 35 have been zeroed in. It is shooting perfectly, 1/2 inch groups at 20 yards centre to centre with Gamo - Hunter dome shaped field use pellets.

I assume the POA and POI were the same? That is, no drop? In that case you can discount the possibility of a slightly bent barrel. The possibility of barrel droop still exists though.

Mack The Knife

[shahid]

POA and POI were absolutely the same. I tried with lighter H & N 14,5 grain hollow point pellets as well. Groups perfectly with the Iron sights.

Checked the Diana 35 again. No barrel droop apparently. But yes now I will try with higher rings and checking out this scope on another rifle, a Diana model 36.

[Mack The Knife]

Okay, let us know how it goes.

Mack The Knife

[Mark]

Well I am not an expert on air guns, so I'll defer to Mack The Knife on the really detailed technical matters but here are some general thoughts I have-

First order as I see it is to pinpoint where the error is. If you can get your hands on one, a bubble level is a handy tool to sort this out. You can mount the gun in a vise and get the body of the gun level, then check the barrel and the mounts too. You should be able to find the problem pretty quickly.

Here are some other thoughts-

Is this a 1 piece mount or a 2 piece mount? You can try reversing the rings to see if that is where the problem is. Mis-manufactured rings are rare but not unheard of.

Now I am not an air gun smith at all, but if this does turn out to be barrel droop, instead of shimming the rings I'd do something to the base.

Suppose the barrel is off plane, and you have used a second scope to verify the problem is not the optics, here is how I would fix this problem-

Since your sights are zeroed, first center the scope optically. Do this by making a cradle (I like shoe boxes, cut 2 V's in the sides where the scope can sit) Put the cradle on a table top with the scope in the V's and rotate it while looking though it, the image wobbles. Adjust it until there is no more wobble and everything is then centered. You can get close by turning the knobs all the way in and out and count the clicks, then put them back 1/2 that number of clicks. Many people will just count the clicks but optically centering them is the best way and only takes a few more minutes (which is usually the difference between a good job and an OK job). Keep the rings on the gun, just remove the top halves.

Anyway, have the gun on a rest and shoot a target at 10M with the iron sights. Now brace the gun that way with sandbags and set the scope in the bottom rings. Use a feeler guage under the scope to raise it up to where the crosshairs line up. You can also go to Brownells.com and they have the mathematical formula to figure it out so you don't need to do it the crude way here. If you don't have a set of feeler guages use some clear plastic tape, then when you have it proper measure the thickness of tape with a micrometer.

Now you know how much error you have. What I would do is remove the base and machine that much off one end so you have the proper angle built into the base so changing rings will never be a problem in the future.

If a machine shop is not handy, I would recommend you make a shim that fits under the base instead of modifying the rings. You can shim it and then fill the gap with a dark epoxy putty and it shouldn't look too bad at all.
You may note that it would also work to measure under the mount instead of the scope, but in practice it is a bit unwieldy

I am not familiar with the Diana 35 but I have a RWS (Diana ) 45 and the mount looks to be easily removeable.

[Mack The Knife]

Well I am not an expert...

Right! Go on then, pull the other.

Completely agree with what you have written. Shimming rings is not something I like doing but many people don't have the skill or the equipment to do the necessary work on the mount.

Perhaps we should send Shahid to Walter....

Mack The Knife

[Mark]

I thought I'd post a little more on this subject, I looked up this link to answer a question Badshah had on sights:


http://www.brownells.com/aspx/NS/GunTech/sight.aspx


And it has the formula for figuring gun sight height corrections, which I will quote here:

Amount of Error X Sight Radius = Sight Correction Needed
Distance to Target

If you are shooting low, you would replace your front sight with a lower sight, and conversely, if you were shooting high, then you would replace your front sight with a higher sight. Add or subtract the number of thousandths needed to the actual height of your sight and you will have the correct height of the replacement sight.

Example
At 100 yards, your shot is 6" low and the distance between your front and rear sight is 19.5".

6 X 19.5 = .0325
3600

Since you are shooting low, you would need to lower your front sight by .032".


I just went and measured the length of the scope base on my RWS 45, which measures 4.717" and I suspect it may be identical the the base on the 35.

To plug this formula in at 10 meters:

10 meters = 393.75"

6" X 4.717"
------------- = 0.0719"
393.75"

So to get the scope zeroed at this moment the back of the mount needs to be shimmed 0.07". However, the scope adjustment is already at the stop so there needs to be a bit more elevation than this. I'd center the scope and measure it again, or.....

Wait just had an idea, be back in a minute!

Ok, I just went and grabbed an empty beer can, took it downsairs, cut it open and put a micrometer on the metal. It measured 0.044" so how about you cut 2 strips of beer can metal, say 1/4" wide and the width of your base and loosen the scope base from the rifle enough to slide them in between the base and the gun, then tighten them down and see if the scope will zero now. If it does, then remove the mount and use an epoxy putty with the shims, put it the entire length of the base, and tighten it down. Put a little wax or oil on the screws so they do not get glued! Wipe off any excess epoxy squeezed out with a piece of paper towel, and do the final cleaning with a cotton swab dipped in white vinegar followed by one dipped in water followed by a lightly oiled swab. After a half hour, turn all the screws out 1/2 turn then back in again. Let sit overnight no matter how short a period the epoxy directions say it is OK to use.

If 2 pieces of can do not work, find a thicker can or add another piece.

Hope that helps, it is not the proper way but should work satisfactorily for you.


Mark

[shahid]

That's wonderful idea Mark. For sure I'll cut up a can and shim it on the scope rail of the Air Rifle and the mount base. Should work I guess. Will let you know of the results asap after the experiments are done.