Sudden 6 Mil Dope Shift? Stumped!

[Nishgriff]

Hi, I don't post here much, and I feel like this is a stupid question, hence posting it here.

I have an AI AT in 6.5 CM, 24" bbl. Before I went to the range today I had 765 rounds through it. It shoots well, best shooting rifle I've ever had. Each day I go out I shoot a cold bore shot at 100, then a five shot group. Then I let it cool, then shoot some dot drills, about 10 rounds. Then I head over to shoot on a longer range, 600-1000. I only shoot one or two groups each day, and they average .35-.45 MOA. Sometimes it shoots .25 MOA, and sometimes .5. This is with FGMM 140 SMK factory ammo.

I have cleaned the rifle only once, at about 450 rounds, just a minor cleaning, following Frank's advice in the online training. It just shoots well.

So today I do the same routine. After my drills I try a box of Winchester 140 Match Factory ammo, and get a .25 MOA group less then .1 mil off my zero. Then I head over and shoot at 600. I get about 20 rounds downrange on steel, hitting with the SMKs in 13 MPH Winds with about a full mil wind dialed in and holding I try some more of the Winchesters, and it shoots really well, slightly different dope, corrected by watching splash, just a few tenths of a mil offset elevation. I'm letting it cool between strings of 10 rounds.

I head down to paint my steel, and when I get back and redope for the FGMM, I 'm pretty far off my dope. I was at 4.2 mils elevation before, and 3.9 with the winchester ammo, so not a big difference. I dial it up and down just a bit, and it keep impacting about two mils up! I try dialing all the way down to 3.5, and no change in impact! I dial down to zero, then back up to 4.2, and I can't see the impact anymore. I can hear it hitting the number boards way up on the berm, so I dial down to zero, and I see an impact 2 mils above the target. At my zero I consistently see impact approx 2 mils up.

This is my second day on the range with a new Schmidt and Bender PM2 (with MTC turrets). I zeroed it yesterday, and confirmed zero today. I didn't do a tracking test, but it was a first round hit with my previous dope at 600. I check parallax. I even checked the torque on the Spuhr mount. All good. I felt like when I was downrange someone fucked with my turrets, though I'm sure that's not the case.

So I went back to 100, dialed to zero, couldn't even get on paper. At least 6 mils up. What could have caused this? When I got home I checked the RTZ screws in the turrets, they were still snug. I cleaned it, it wasn't all that fouled. I thought maybe it was the RTZ screws coming loose or something as I dialed and it moving the turrets but not changing the erector. But the screws were still snug. I mean, if it was fouling, wouldn't it slowly open up, and would it get THAT far off? I mean, it's more than six mils off at this point.

I'm stumped, ideas?

 

[308pirate]

Your scope broke

 

[Deleted member 113831]

That is a scope issue. No cleaning or ballistic problem would ever cause that much of a shift or that suddenly. Something in your optic or mount took a shit.

 

[Nishgriff]

That is a scope issue. No cleaning or ballistic problem would ever cause that much of a shift or that suddenly. Something in your optic or mount took a shit.

Fuck. I assumed I was doing something wrong, didn't even cross my mind that this $3000 optic would be defective.... Have initiated a return on it... Thanks

 

[Rocketvapor]

Stabilize the rifle and do a tracking test on a ruler (or rule )
Got a mil ruler?

 

[TACC]

I would like to hear what the factory says happened to the scope, if they tell you.

 

[hlee]

I had an issue with a vortex scope with a similar phenotype. It turns out that I had not torqued down the screws that attach the turret knob to the turret stem sufficiently, and the knob was- slowly- moving off my zero every time I dialed elevation.

 

[Nishgriff]

I had an issue with a vortex scope with a similar phenotype. It turns out that I had not torqued down the screws that attach the turret knob to the turret stem sufficiently, and the knob was- slowly- moving off my zero every time I dialed elevation.

Thanks for the idea, but I already checked those... I can't find a torque setting for them, but they are just past finger tight with a small hex wrench. Anyone know a torque value for the RTZ set screws?

 

[hlee]

Thanks for the idea, but I already checked those... I can't find a torque setting for them, but they are just past finger tight with a small hex wrench. Anyone know a torque value for the RTZ set screws?

I figured you might have, and I’ve never seen a value for them. I do what you do; as tight as I can using the small end of the hex key for leverage. I’ve come to appreciate a splined turret stem, even if the hash marks don’t line up 100% at every setting...

 

[Hollywood 6mm]

Something very similar happened to a friend of mine at a match last year with his PMII - it was a 2+ MIL horizontal shift that would come and go for no obvious reason during a stage. Sounds like something's out of whack in the optic.

 

[shoot4fun]

I had an issue recently where the gun would not put two shots even remotely close together. It started suddenly that a gun that normally is capable of 1/2MOA accuracy at 500 yards was now printing 4 1/2 MOA. One of the top caps on the front ring had come loose. Yes, they were indeed some high quality rings but apparently the end user was able to take them down a notch.

 

[bax]

Put a known-good scope in the rifle and shoot your good ammo. It better shoot little. If the new scope shoots little, it was the scope you took off. If it doesn't shoot little, check the base, receiver screw torque, receiver bedding, barrel and brake are tight -- all the ordinary stuff. If you think the problem is the scope you took off, check the ring base nuts and the ring screws for proper torque.

After you make an adjustment, tap both scope turrets. It is possible that twisting the turret doesn't cause the reticle to move because the gizmo inside the scope that carries the reticle is sticking. I think this is called the erector. When you tap the turret, the erector moves and you get the adjustment that you requested (when you moved the turret). I suspect it has to do with the temperature and the lube the scope guys use and how old the lube is but that is complete speculation.

Suppose that you make a .2 mil change, don't tap, shoot, get a hit in the old place, shoot again and get a hit in the expected place. Make another .2 mil change, tap, tap, shoot and you get a hit in the expected place. Make another .2 mil change, don't tap, shoot, hit in the right place. In the first case, the erector was being sticky so the reticle didn't move, the shot caused the reticle to move then the erector settled in the right place. In the second case, the taps relieved the stress, and the reticle moved. In the third case, the reticle did not stick - it doesn't stick every time. If this little test shows a sticky reticle, it still needs to be fixed but it helps the scope guys. I would use a round of ammo and just give it a light tap with base of the round on the end of the turret. I did this with a guy's scope two weeks ago and it worked.

 

[Rocketvapor]

Stabilize the rifle and do a tracking test on a ruler (or rule )
Got a mil ruler?

Do you even have to shoot to check tracking?
"So I went back to 100, dialed to zero, couldn't even get on paper. At least 6 mils up. "
It's probably too late but I wouldn't touch any screws until I ruled out the scope itself.

 

[Kadams1563]

Those MTC turrets are the problem IMO

You know those three screws on the top turret. Well your not supposed to un screw one of them.

Which in you say? Have no idea because they ALL look the same and they are the same Allen key.

Two friends have had problems with that scop and those turrets.