I have a BCM complete rifle (2013) that has never had anything on it other than backup sights. Recently purchased a Eotech EXPS2-0 and after doing some cleaning noticed the sight had shifted toward the stock end of the gun. I had mounted it tight (to where it would not slide by hand and felt sung) against the muzzle end of the rail section and added one more click to the adjustment screw. Still if send the bolt home a half a dozen times I can see a shift away toward the stock. Any tighter I am afraid I will damage the QD lever…is this normal with sending the bolt into battery but stays in place at the front end of the rail section during recoil? I would think that it would stay put but this is my first rifle and optic. Thanks
I wouldn’t worry about it. I’ve had two uppers that required the EXPS to have a few adjustments before it stayed tight. Others it was tight & never moved right from the get go.
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Is this due to finish wear? I do notice that it is showing rub marks on the sight and the rail hasn’t lost finish, but you can see where pressure has been applied. Finish wear doesn’t bug me, a sight that moves when releasing the bolt catch does. Again another click on the adjustment screw feels like I have to put decent leverage into it.
I’ve always attributed it to a slight crush of the rail although I never measured anything to confirm. Coincidentally, the two uppers I had to readjust the EXPS to were also BCM’s. I had it on a Colt for over a year first and never had to readjust it after the first initial mount. It wasn’t until I played musical RDS that I noticed it required readjusting on those two particular uppers. All my other uppers are fine. I’ve since settled into a configuration that I like & it hasn’t moved since. Don’t worry about the added pressure required after the adjustment. It shouldn’t require anything more than just your hand to snap it in place but it may take a firm push to lock it in.
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Shim with a little foil, mount the XPS all the way up in the rail slots, apply vibratite to thumbscrew , allow vibratite to dry, torque that sucker in there. My XPS hasn’t moved a nm since I installed it.
The OP has an EXPS, not an XPS. Different mount. The EXPS has a QD lever lock & no thumbscrew.
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Oh my bad, I missed that. Still, shim, vibratite, and there must be a tensioning screw on the QR. Vibratite that.
Once locked in place, there’s nothing to rotate. The adjuster has detents. I’d advise against using any thread locker on the the EXPS system.
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I saw this with one of my .30 caliber AR’s when I installed a 1x4 scope on a QD mount. Every time I removed the mount, I noticed the same thing you are seeing. I did not notice a zero shift out to 300 yards. I placed tin foil between the mount and upper receiver, shot groups and did not notice any difference in zero, so I removed it.
Adjust the tension on the throw levers until the mount does not appear to move when cycling the charging handle. Lay the mount on top the carbine, slide it to the rear, then throw the levers to tighten it. Unless you are shooting at 500 yards or farther, it should not make enough difference to have a noticeable affect accuracy.
I don’t use thread locker on the QD mounts.
Ahhh… I only have the XPS… I always lightly vibratite optics, grip screws, and ambi safety lever screws. Super easy to remove but holds things in place. Sorry for the bad advice.
If I understand you I will have to let the rail crush until it “sets” with this particular sight and keep adjusting the screw to make it do so? So far it is adjustable with one hand but the next click from the detent will require a good amount of force. I know the adjustment screw is there for different tolerances among rails, but this is normal you are saying to have to keep adjusting?
So if I understand correctly the rail needs t
When mounting anything in the military on a rail, especially an optic, we always get it on the rail and in the slot. Then lightly push it rearward until it contacts the back of the slot. Then tighten it down. The shift is common under recoil and even closing the bolt, but if it is pushed to the back of the slot, it will not have anywhere to shift to. This maintains an accurate zero. I would tighten it a little by following the process above and you will be good to go.
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By push to the rear do you mean the portion of the rail closest to the front sight or to the buffer tube? I have mine pushed to the front sight side of the slot when tightened. It appears to shift toward the buffer tube when cycling the bolt, but maybe when shooting a mag the amount of recoil force cancels out the bolt cycling.
I’ve only had to readjust a couple of times before it remained in place. It shouldn’t require tools but quite possibly two hands.
FWIW, based on my experiences & what I’ve read here, optics should be pushed forward in the rail prior to tightening it down.
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This would be my first time adjusting it one click tighter after initial fitting when I got the sight. I also heard here and elsewhere that it needs to be pushed forward
Slow work day so I called Eotech. Paraphrasing … but the rep stated it should not be moving and as long as I do not need tools to put it on and remove it I am ok, but also mentioned on some rails if the finish is thicker the sight may need to compress that down for it to settle. Maybe BCM has thicker coatings on their uppers?
I have also come across the comment that the mount should be pushed forward before tigntning down. We could be talking a very slight movement here but–Any opines one way or the other as to why?
In the simplest way I can explain it…
Recoil has a rearward and a forward impulse. You should push the mount toward the muzzle when installing to support it during the forward impulse. When mounting toward the charging handle, the mount MAY work itself forward during the forward impulse.
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The mount seems to work it’s way back as you fire the weapon, so I like to push the mount to the rear before tightening. That way when I zero an optic, nothing should change when I remove and reinstall the mount.