This is STRANGE so please bear with me for a little background...

In 1994 I purchased a Remington 700 "Varmint Synthetic" in .308 Win. After some work on the barrel channel it shot reasonably well (1" at 200 yds) but it should have done better. The chamber throat was loooooong. Remington confirmed this was by design "If you wanted a match chamber you should have bought a 40XB."  So it was relegated to the safe.

A couple of years ago a friend started bugging me "we can make this rifle shoot."  He had some favorite .308 handload recipes etc. I added a Timney Calvin Elite two stage trigger (NICE!) and a Leupold FX3 12x target scope. Bottom line, it shot like crap. Much worse than 20 years ago.  Not worth rebuilding - might as well just purchase a new rifle.  Considered a NM M1A.

I had never shot an M1A with a scope so I added a Sadlak mount to my 1990 Springfield Armory M1A and mounted the 12X scope.  Even with my home made (foam, duct tape and an old sock) cheek rest... Results were OK but the standard stock and a scope?  No. Perhaps with a chassis system.  For my next trick, my step son said "why not a .308 AR?" Because they are not standard I said.

While that is true strictly speaking, a lot of manufacturers follow the DPMS pattern.  I purchased a Fulton Armory Tital with a 24" match barrel.  I mounted the 12X scope with a Leupold Mark 2 IMS mount. The first shot at 100 yards was about 4" high and 4"+ right. Down 16 clicks and left 16 clicks.  Much better. A couple more clicks left. Better. A couple more clicks left - 3" left and about 2" high. After that - all over the place. Tried various bullets (Sierra match, Hornady match, various ball and soft points) and powders. A nice shotgun.

I replaced the 12X scope with an ancient Leupold 6.5 - 20 EFR target scope.  Hornady  150 gr ball (not match) and WW 748 powder - 7 of 10 shots in a 3/8" hole - 6 of those in a small part of the hole. The other three I pulled I think.  So the rifle can shoot.  

Back to the 12X scope it seems that the eye shell was loose from the lock ring. Short of tightening it with Vice Grips and a strap wrench the thing would come loose after a few shots. The eye shell has a few thousandths wobble and combined with 12x... of course it is all over the place. After several "discussions" with Leupold tech support (who never heard of this problem) the scope went back.

It seems that Lepold used what is caller a "multi-start" thread on the eye shell.  This is basically several threads cut in parallel at the same time. Think of dragging your finger nails around the scope tube. The result is a small pitch (the distance between the thread grooves) but a large lead - the amount the eye shell moves with each revolution. This allows for quick focus adjustments but poor clamping force against the lock ring.

The 12X scope came back thoroughly "inspected and adjusted."  I mounted it on a 16" pencil barrel .223 carbine.  5 shots and the eye shell is loose. A bit of gorilla tightening and another 5 shots. Loose again. Back to Leupold again - this time at their expense.

I found an FX3 25X target scope on Cyber Monday sale at a price I could not resist. Put it on the Fulton - same issue. Back to Leupold. And they were both returned to me  thoroughly "inspected and adjusted." The 12X scope went back on the .223 carbine - no sense wasting .308 ammo. Tightened the thing with a strap wrench while I had it on the bench. 7 shots and the eye shell was flapping in the breeze. Back on the phone to Leupold - I want to speak with a manager.

Eventually I got hooked up with a senior customer service rep. He sent a call tag for the scopes and called me when they arrived. He tested the scopes on an AR at their test range and confirmed the issue.  "I don't know why the used such a thread design on a target scope...If we can't get these fixed I will put you in some other scopes."  I ended up with two VX3i scopes. Nice 30mm brutes. But with the same thread design  I put an 8x-25x on the Fulton. The lock ring on the 30mm tube seemed a little more secure and I think it only came loose twice in the course of several short range sessions. I put this down to not tightening it sufficiently (perhaps.)  

As I needed some more practice I decided to move the scope to a .223 AR.  The rifle I used is my light weight match rifle - 20" pencil barrel, aluminum free float tube, flat top receiver and normally my home build match iron sights.  I had just torn the thing down to do some mods and thought I would do some accuracy testing and load development before I re-installed the match sights. Wrong!  A few shots and the eye shell/lock ring are loose. Repeatedly.

I sent a message to my contact at Leupold.  He called me back, sent a call tag for the two VX3i scope and will be sending me two VX-6HD 3-18x50mm scopes. They have a different  focusing mechanism. The eye shell does not rotate. I looked at one of these scopes at Cabelas. WOW!

In summary... Leupold customer service has been GREAT and GENEROUS. Thank you!!! (I still think the multi-start thread design was dumb.)

And finally my questions to the community...

Has anyone else experienced this sort of issue?  It appears that Leupold uses the multi-start threads on most of their scopes these days.

Secondly... does an AR platform rifle produce some strange accelerations, vibrations. harmonics which might be causing this issue?  In hind sight I think the loose eye shell contributed to the poor performance of the Remington 700.  However, I did not notice it at the time and did not go back and try again.

I recall reading many years ago about a scope which was advertised as as safe for use on an air rifle. An air rifle? That has no recoil to speak of. But in fact, the air gun referred to was spring powered and apparently spring powered air rifles produced a characteristic acceleration impulse which would tear up a scope which could stand the recoil from a.416 Rigby. It makes me wonder about the AR platform with its recoil spring, buffer etc.

I offered to send the 16" .223 upper to Leupold if they wanted to do some testing on it.  I will see if they take me up on my offer. I guess I could get another 12X scope, some accelerometers and interface them a laptop and do my own research. No, I think I will leave that to Leupold.  I may need the laptop to operate the VX-6 scope. Any scope which requires a battery is too complicated for me.  All I wanted was a simple, single power target scope

Ken

I've got a Leupold Mark AR 3-9X40 mildot scope I have mounted on a 16" 6.5 Grendel.  The Leupold ocular adjustment ring continually came loose.  I tried tightening but it always came loose.  I never adjusted the focus on this scope after tightening but I guess the recoil was causing the adjusting end of the tube to come loose.  I finally used some white thread tape on the scope tube threads and tightened down the adjustment, ocular end.  I did this a year or two ago and it hasn't come loose since.   I don't really like the design but otherwise the scope has been great.  I have been around several other Leupold scopes that never exhibited this problem but most of those scopes were on bolt actions which weren't shot hundreds of rounds like my AR-15's

Thanks VASCAR2,

I have two of the baby Mark AR 1.5 - 4 scopes mounted on .223 ARs. They are middle and heavy weight 16" barrels.  The lock ring on the Mark AR scopes is I believe a bit thicker (more threads) than on the other scopes. I have not noticed the problem with the Mark ARs but I am not sure I have used them since this fiasco started thus I was not necessarily looking for the problem.

Ken