I have no explanation for these rather drastic groups from the range trip a few days ago.

I use a Leatherwood ART 2/600, 3~9X40 scope on the very accurate No4 Mk2. It normally holds under 2 MOA with 150 gr ammo which it seems to prefer.

Anyway a small description of how it works is in order before I get into the gory details.

The scope tube is a fairly conventional 3~9X40 zoom scope with a specialist reticle. It has 1/4 MOA click adjustments for windage & elevation.

It's integrally attached to a spring-loaded base that has a pivot at the front & a spring-tensioned bearing for the cam surrounding the optic tube at the rear. The base is both a mount adapter for the rifle type & the other half of the shooting system of the ART.

There are 3 adjustment rings surrounding the rear adjacent to the zoom power ring.

One is a "Power setting" ring, Basically a calibrated B.C. scale.

Another is the "Range setting" ring where you can either dial in manually, or set automatically via the scales of the reticle, the intended firing distance. (more on that later)

The third is simply the zoom setting ring from 3X to 9X mag.

The  "Power setting" ring you look up (or calculate from actual firing data at distances) the ballistic properties of your load. This links to the "Range setting" ring, so it knows the bullets path.

The two together determine where the spring-tensioned circular cam at the rear sits at a given distance as dialed by the Range setting.

These two are locked together except for you using a different load for some reason.

The zoom power setting is normally used "coupled" to the other 2 rings.

To determine & set for a desired range on target you simply rotate the zoom (& the the other 2 rings) which are coupled to it, till a known size target matches the reticle.

This also compensates for trajectory at that range so you aim dead on by tilting the tube bearing against the base.

It works very well as long as you set the 3 rings up carefully, just dial & shoot. As you zoom to fit the target into the reticle the rotating of the three coupled rings pushes the cam against spring tension tilting the entire optical tube by pushing against the fixed mount/base section.

The one drawback is in "coupled mode" your power zoom setting is tied to the firing distance.

So there is an "uncoupled mode" available. This allows you to use any magnification at any range, but you lose the "Automatic range-finding & Bullet Drop Compensation" features.

Anyway. I decided to try Uncoupled mode just to see how it works IRL.

Theoretically you can start "coupled" , use the BDC  feature, then uncouple & change the zoom interdependently of the other features while retaining most of the functionality of the ART systems "Camputer" features.

You zero the whole thing at 200yds. Because of this I wanted to see what happened if i set the Camputer to "2"oo yds, which is 3X mag), Uncoupled & zoomed to 9X.

It was a disaster!

 

Groups opened up to dreadful 11" for 10 rounds & the "groups" were almost 6" low.

I didn't have a chance to re-couple & see if the problem went away or not.

I have checked the obvious places for trouble, the rifle is still properly bedded & functioning normally, the adapter (mount) is secure on the rail & the rail is secure to the receiver. The tilting pivot & spring are fine, lots of tension forcing the cam into full contact with the rail. The reticle seems normal. The optics seem secure nothing is visually "off" & there's no sign of anything loose in the optics.

I'm at a loss to explain the sudden complete collapse of the system to such a huge extent.

My only suggestion is to remove the scope and mount and check with the aperture sights to rule out any issue with the rifle. 

A change in the POI might be expected as the zoom is uncoupled from the BDC but not a loss of accuracy.

This is odd.

I replied to a couple of these posts & the reply is missing?

Is anyone else missing any recent posts?

Anyway I have checked the cams, springs, looked for dirt in between the 2 sections of the tilting mount & double checked all the cam & other things are secure & correct. I even clamped the scope into a rest & zoomed between 3X & 9X & back again several times looking for any movement in the reticle due to optical issues.

Still nothing!

Sigh.

Yes it was cooler this time. 45℉ as opposed to 71℉.

I should also mention one of the 2 groups was fired with the exact same ammo from before a 150gr powered by IMR 3031 & the other a slightly mild 174 Gr load fueled by IMR 4895 as I'm starting to work up a load for this combination.

I could see the lighter load with the heavier bullet printing low, that was part of the test. But BOTH were equally low & the massive expansion of the group size is unexpected.

We used to call thing like this "gremlins", particularly if they also vanished without explanation as suddenly as they appeared!