Complete BCM 14.5 KMR upper with about 1500 rounds through it. This upper has had an Aimpoint T1 zeroed at 100 yards with no problems.
Today I mounted a brand new ACOG TA31F w/ green chevron on a TA51 mount from Optics Planet. I was using Winchester white box 556. I am still about 5" high and 1" to the right at 100 yards and the turrets are maxed out. I didn’t force them once they got tough to turn as I realized something was wrong and just stopped for the day. At this point I am stuck until Tuesday when customer service opens up so any suggestions are appreciated.
Make sure the mount is tight on the rails, as well as the acog to the mount. Are you giving it a little tap after adjustments? Just make sure everything is tight.
I’d say check the mount as well. I’ve felt really stupid having the same issue and realizing I never righted the rings on the mount.
I was always told to tap the ACOG after making adjustments to make sure they settle in. I don’t know how valid that is, silly habits travel fast in the military.
Have you boresighted it? I would run the Lazer on and experiment with taking the sight on and off and make a few adjustments and see how your adjustments affect the position of the dot.
I know this may be a wild ass question, but it may be worth asking, since I have seen some wonky shit, is the mount for the ACOG completely on the upper receiver or are you bridging the gap with the rail? I know not likely, considering the eye relief, and the fact you would be getting your cheek weld on the upper’s rail, but it still bares asking since that has been known to cause some issues. Second question, are you running a rear sight under the ACOG, and if so what sight? I know when I first bought my ACOG I tried running my old ARMS #40 under it and I could get the ACOG to mount, but I was making contact with the rear sight and I can see that possibly causing issues, though I never fired it and swapped it for first an MBUS and then a Troy, and in the future may go to a Magpul Pro or KAC, haven’t quite decided.
Other than that, as others have said, check the mount and make sure it is on there right, though the mounts are pretty grunt proof I am sure you can manage to jack it up, and if that is the case per chance don’t let is sour you for the mount since they are pretty damn solid. And to repeat others as well, I would mechanically zero the optic, and make sure to tap, rap, or smack it to make sure the prism is settled. That was discussed here recently and is something that the prism of the ACOG does sort of require and can cause groups to act weirdly and have you chasing a zero.
In my experience optics bridging the handguard receiver should be avoided at all cost. If your having BUIS clearance issues checkout the macedon defense KRAM. It comes in 3 heights and it also can move the ACOG rearward if desired. I dont think they sell them direct but their websight has some good reading on selecting the proper one. I got mine from weapon outfitters. Im not affiliated in anyway just a happy customer who learned about the KRAM on here a few weeks back.