scope loseing zero (lapping)

[evo]

for anyone interested I found this and it certainly makes an interesting read, hope you found it as interesting as I did

 

INTRODUCTION

For years I have seldom gone from one shoot to the next without having to adjust my zero. I often wondered what the cause was - cheap scopes I thought. Sometimes on hot days my zero would change during an FT shoot and 2 or 3 shots were missed before I realised what was happening.

I recently purchased an expensive Burris 8-32x R/A scope and I still suffered the problems. I emailed Burris and they recommended lapping my scope rings or buying their Signature rings with floating inserts. www.burrisoptics.com - this is a superb FT scope by the way, pin sharp image and great range finding.

A quick web search using "scope ring lapping" via www.dogpile.com revealed all.

This procedure is well documented and understood in the US but seems to me virtually unknown here in the UK.

The rings on your scope mount are seldom, if ever, (especially cheap mounts as we tend to use) perfectly round or lined up. As a result they cause the scope to distort out of shape and the scope is affected in 2 ways:

1. The kinking causes the internals to snag and therefore when you dial or focus (parallax) things don't always return or move to the desired position.
2. The kinking causes the scope to bend so it isn’t perfectly straight. When it expands/contracts due to temperature changes it moves down/up or sideways. If it is lying perfectly straight it expands/contracts in a straight line which is ok and results in no zero loss.

The following is largely taken from US sites combined with my own limited experience.

Proceed as I did at your own risk - it worked well for me

SCOPE RING LAPPING (instructions)

Test the rifle with the scope mounted as you want it, pack the mounts as required in order to get the turrets more or less central (optical centre where the scope’s internals and optics function best). Mark the scope’s position with Tipex.

The best packing material is a pop can which are made from very thin aluminium about 0.12mm thick, cut into suitable pieces with scissors (care sharp edges).

Low shots = raise rear mounts

High shots = raise front mount

Shots left = pack front mount on left (vice versa for rear)

Shots right = pack front mount on right (vice versa for rear)

Try the rifle after packing and fine adjust as necessary with extra or less packing.

You can use a calculation to work out how much packing you need

Distance between scope mounts X distance shot off target

Distance to target

Pack for windage or elevation one at a time

Be sure to use the same units, preferably mm

1 inch = 25.4mm

1 yd =36 inches

therefore 30 yds = 30x36x25.4 = 27432 mm

E.g.

If, at 30 yds your shots go 5 inches low and your mounts are 4 inches apart (centre to centre) the calculation would be:

127x101.6 / 27432 = 0.47

So in this case you would try packing four layers under the rear mount

0.47 / 0.12=4

Do not remove the scope rings now you have them set up

The lapping should be done with the mounts on the rifle after setting up/packing etc

Clamp the rifle into a vice with soft wooden clamps, in or out of the stock

Remove the top ring halves and remove the scope

Wrap the gun/stock parts under the rings with paper or old towels

Put course car valve grinding paste into the ring halves - top and bottom

 

Lapping tool (seemingly only available from US). Making your own is easy. One inch steel bar 15" long, handles nice but not essential.

Put the lapping tool centrally into the bottom rings as shown

Very carefully move the steel bar back and forth a couple of minutes

Check progress and when all the black is removed (at least 90%) the job's done

Put the top rings back clamping the lapping tool into the rings but don't clamp too tightly

Rotate the bar until it feels loose then start going back and forth. Keep doing this intermittently tightening the clamping screws – this takes a bit of time

When complete remove the lapping tool and remove the paste from the rings - WD 40 works well

Wash in hot water and Fairy Liquid to remove the WD 40. Allow to dry or use a hair dryer

Now you have a choice, stop there and try gun out over time or continue as follows:

Grease the scope where it contacts with the rings, grease the screws, grease all parts you don't want to stick together

Load Araldite Rapid into the bottom and top rings and clamp down the scope and allow to dry until it gels up. Remove the scope and trim off the excess glue and re-clamp the scope ensuring there is still grease on the contact points - now you see what the grease is for, the Americans sometimes use what they call Gloop (Latex based glue from model shops)

Allow at least 24 hours to dry properly

When mounting and clamping the scope into the glue watch out for the excess oozing out and running into places you don't want it to go

You can buy Burris Signature scope mounts that have floating inserts just for this sort of problem. You can also buy from them off-set inserts for skewed barrels etc.

All Burris stuff can be obtained from this shop and from what I see on this site at very good prices too. http://www.lsbuk.com/lsbburrispage.htm

Memo December 2008 - Steve's link no longer seems to work so I would suggest that you try the Burris UK website.

Remember:

If you re-position your mounts you will need to re-lap them

If you re-pack your mounts you will have to re-lap them

One piece mounts are easier because you do not need to re-lap after packing or re-packing

Never pack inside the rings, always pack between the mounts and rifle mount receiver

Scope ring alignment pointers

You can easily make your own but the points must be turned accurately on a lathe.

The pointers may line-up without lapping, this does not mean lapping is unnecessary

After lapping the pointers should line-up perfectly or very very near

1 mm of scope movement lateral to the sight line = 9 inches at 30 yds, therefore 0.1mm at the scope = 1 inch at 30 yds

Cheap scopes can snag inside even with lapped rings. It is recommended when dialing in, you over shoot the mark and return to the desired setting

 

cheers Evo

Edited August 26, 2015 by evo

[Underdog]

Yep, done it a few times.

Good article, and good of you to share.

[evo]

cheers UD , very interesting read and a good explanation of " what to do " if this happens

[bruno22rf]

Might be of some use if you shoot over very long ranges but for Airguns-cannot see the point-as for the mounts preventing the scope adjusting correctly-the interference fit is about the size of a gnats man sausage and normally well away from the adjusting mech. -unless you use 1" mounts on a 30mm scope or your scope is made of Jelly I doubt that the adjusters would be affected by a bit of pinching?

[evo]

1 mm of scope movement lateral to the sight line = 9 inches at 30 yds, therefore 0.1mm at the scope = 1 inch at 30 yds

 

Bruno it certainly looks good to me if the above calculations are correct and if it only helps one person to sort out his scope then all,s good, its pretty obvious that its not just for powder burners but the calculations that have been used are solely for air rifles which is why the gentleman used the lapping method,

 

atb Evo

[bruno22rf]

.1mm sounds impressive but means nothing in real terms-if your scope is moving around that much then you are either firing a high powered rifle with cheese mounts or you have forgotten to tighten your mounts-in which case you can lap them till the cows come home-if you want ultimate accuracy from a scope then far more relevant is its position in relation to the line of the bore- You can also lap your mounts till your arm falls off and it will not make a blind bit of difference because the manufacturer holds all the cards here. While considering this you also need to know that ,on many guns, the scope mounting grooves were machined true to the bore. Finally-for those worried about scope movement-unscrew your scope rings then re-tighten them using only your thumb and forefinger on the screwdriver-now try and twist the scope-not easy even at this light torque-impossible when correctly tightened unless your mounts are total carp.

[Les*1066]

Plenty of good info there.

 

I've used broom/brush handles wrapped with wet-or-dry paper quite successfully as lapping tools.

[Bewsher500]

get better mounts and scope.....