I have a TA31A4RCO that I moved from one rifle to another. I had it zeroed perfectly on the other rifle before moving it over. Both are similar rifles, the only difference being one has a gov't A2 profile 20" bbl and the other has a skinny pencil A1 profile 20" bbl. I thought it should probably be pretty close and should be pretty easy to get it zeroed but I was wrong.

I wasn't even on the paper at 100 yards. So I zeroed it at 25 yards using the 300m mark. Went back to zero at 100 and once again I still wasn't even on the paper. I was having a bad day not figuring out why I couldn't zero this thing so I left the range. It just doesn't make any sense to me because I zeroed it on the first rifle in less than 10 shots. I wasted 40 rounds with this thing and couldn't even get on paper.

I went home and thought I should try to optically center the reticle. Trijicon manuals have no information regarding how much max moa adjustment in clicks this scope has. So I tried the easy mirror trick to center the reticle but the only problem is you can't really with an ACOG because the front tube isn't flat, it's hooded. So I had to just do this the hard way by counting clicks. First I maxed out windage and elevation then I started counting clicks, almost lost count because I counted 180 clicks! That's 90 MOA of adjustment, to me that seems rather excessive for a scope that you're supposed to just set it and forget it. Anyways I backed off 90 clicks which should theoretically bring it back to center.

I hope I didn't F anything up and have to send it back to Trijicon for repairs. Their manuals are not very clear on something like this. All I know is it worked perfectly on one rifle, and not on the other rifle. I will go back to the range this Sunday to try again and pray that it works this right this time.

And yes, I know to tap on the turret to set the prism.

I think you should try going back to the 25y range and zero it 1.5" low.

Then go back to 100y.

I think your first big mistake is expecting the zero to be really close from one rifle to the next.  Definitely not always the case.

I'd make sure the mount is tight to both the optic and the rail, then go back and do as mentioned above,  get as close as you need to to get on paper, adjust the zero 1-2 inches low there, and that should put you on paper to fine tune at 100yd.

Yup, it's mounted tight with blue locktite to the TA51 mount. Mounted the TA51 to the receiver hand tight and then 1/4 turn with a flat head.

I believe with the ACOG (I think) it's mentioned in the manual somewhere to not go to the extreme ends of adjustment to count clicks to find center. It will damage the adjustment mechanism or something.

Sounds like you might end up needing to send yours back for repair.

Shit! You're right. Well I dialed it until I definitely felt a wall with resistance and I stopped there and didn't dial it any further passed that. I hope I didn't break nothing. 180 clicks does definitely seem like way too many clicks though. I thought these had a reputation for being bombproof, why would dialing it to the wall damage it?

It's not a scope that's really meant to be dialed, just zeroed and then use the reticle for holds.

I don't recall that max adjustment range warning from the manual, but if it is... maybe an oops.

I'd still try again to zero it starting at close range before sending it back.... if it's broke, I bet trijicon will fix it though.

As someone who has owned at least one ACOG for over 15 years (granted they have sat on the shelf more than they've been used) this marks the first time I've ever heard the phrase "tap on the turrets to set the prism".  Googling that, I do get some hits, but... not something I've ever had to do, doesn't sound like something I would do, and if "tapping the turrets" changed the zero for me, I think I'd be sending it back to get fixed... just my initial thought on that.

"I went home and thought I should try to optically center the reticle."
A little known dirty secret with ACOGs is they track like absolute garbage, and they don't care about optical center\eyebox.  When you turn the turrets like that all the way to one side, and then the other, there's a good chance the prism got stuck or shifted around in there in relation to the turret clicks. Which is unfortunate, because 50% of the time ACOG optical center doesn't line up very well with your particular rifle's centerline\rail very well so you have to crank on those turrets 40 or 50 clicks to zero the thing.. at which point your eyebox is all jacked up to one side.
Try zeroing the scope again, do a rough bore sight first on a rest to get you close. If you have to crank all the way over to one side or up\down, return to Trijicon under warranty and in the comments "not enough adjustment to zero" and they'll most likely replace the prism with one that is better centered in the scope body or other repair magic. The return\repair process is easy and doesn't take all that long to get it back.  At least that's what I've had to do, they won't do any warranty work if you can actually zero the scope, even if you're one click away from maxing out adjustment.  I have a couple ACOGs I can't use because while technically functional\able to zero, they're optically unnaceptable because the eyebox and shadow looks like a half moon if not perfectly centered behind it, Trijicon won't fix these.

It's in the manual.

Don't take this the wrong way, but I would know whether an optic was broken. You're questioning of it leaves me thinking there may be some user error. This is how I zero my optics on ARs.

So what I always do is take the upper off and put it in a tripod. I remove the bcg. I have a hallway that is about 20ish feet. I put a target at the end of it and set my tripod up acrossed from it.

I line the bore up with the target using my eye looking down the barrel. I then adjust the scope to be about an inch and a half high.

That gets me on paper at 50 and usually I just need to move a couple clicks.

You don't need a tripod. Lay the upper flat on a table. Hang the target on a wall across from it and align the target with the bore. Adjust the scope like above.

I never shoot more than a few rounds getting rifles zeroed with this method. Very simple and straightforward.


Edit: why were you holding the 300m dash at 25y? What zero are you trying to have?

Learn something new every day!

I've never had to do more than move the turrets a few clicks off factory center to zero an ACOG on any given rifle, just like any other scope.

I agree this sounds like a good chance of user error.  On the 300yd mark zero at 25, without running numbers, I think that might put your 100yd impact way high vs. the 100yd zero point in the reticle.
The above idea of boresighting to get close is great.  I would definitely do that.

What I normally do with a new rifle/optic combo is shoot a single round at the center of a full size IPSC at 50yd.  That gives a lot of leeway to have it on paper.  (If the adjustment is 22MOA adjusted in some direction now, maybe not, but normally this applies).  From there in a MIL or MOA hashed scope reticle, it is exceptionally easy to just look at where that single shot is, dial the impact to very near zero, confirm with a second shot at 50, then move to 100yd to fine tune.

An ACOG reticle won't let you be this precise on the second shot, but if it is 1/4 or 1/2 MOA clicks (I forget), and even if it isn't tracking perfectly, you can still estimate the distance between your POI and POA at a large target at 50yd based on one shot, adjust that amount, and be fine tuning within, say, 5rds.  If you're totally off paper at 50, move to 25 and try again.  Depending on the target size, a bigger piece of "paper" might help.  But you should be able to address this via boresighting before firing another round, as mentioned above.

I'd just walk up and shoot the target to make sure you're on paper relatively close.

Walk back a bit, fire and make some adjustment.

Walk back more, fire and make some adjustment.

Repeat until you're near 100 yards.

At 100 yards, fire and make adjustments until you're zeroed.

Boresight it and get on paper @ 25. Zero 1.5” low @ 25. Fine tune @ 100.

It’s been covered, but hopefully maxing out the adjustments didn’t hurt them. Usually that comes from cranking in them, but still the same. This will absolutely break the shit out of an ACOG, which is an otherwise durable optic.

They track awful. They adjust awful. C_Does covered this on YouTube and documented it pretty thoroughly with his sample size of one. This got covered here but everyone got butthurt and said he was stupid because reasons.

I don’t think it matters much. It’s not a dialing optic, but it’s still a thing, and you’re encountering it right now. Cross your fingers that you didn’t damage it while discovering this.

the only reason it tracked awful in his video is because he was cranking down on the adjustors and zipping through the clicks as fast as he could. The Army ACOG manual specifically states to make the adjustments slowly

As much as I like C_Does his ACOG review did leave me scratching my head a few times. Saying you have to get right up on the TA33 like 20 seconds after he walks back like 10 feet and can still see the reticle for instance.

The way I zeroed it before on my other rifle was,

zeroed the 300m mark at 25 yards,
then zeroed the 100m tip at 100 yards,
then finally fine tuned the 300m mark at 300 yards.

I did most of my shooting at 100 yards and I was shooting 1.5 MOA groups.

One thing I just remembered is my A2 flash hider spun itself loose. I think it wasn't timed perfectly right from the start. Could this affect the accuracy?

I did notice it though however, and I tightened it back on. I'm bringing my other rifle with me too on Sunday to see if maybe it's the barrel and not the scope. The upper I used has a new barrel professionally installed.

I have one that has to be tapped. I just recently changed out the mount and rezeroed, and that was a factor.

Yes

Okay that makes sense, but even after I fixed it at the range I was still having problems. I eyeballed it but it looked perfectly straight.

To repeat what others say - ACOG's are tricky to zero and adjust.  They track awful, and then settle.  Also, YES your loose flash-hider WILL throw off shots.  I speak from experience on that one.

If it were me, time to walk it back to just 25 yards and a big piece of paper, and move forward from there.   Tapping the scope with every adjustment.  And not lightly.  Then mag-dump, jostly and bang the rifle around.  Then confirm zero.  Adjust and repeat if that's not your zero.

My ACOG came on a rifle that likely was mounted and abandoned for close to 20? years.  I only had 10 clicks or so in either direction before it got sticky.  So I said F'it and forced it past that with plyers.  Yes, plyers.   Then it started working.  Now it's zerod.  Let's just say if I had paid retail for that ACOG and it was relatively new, no way would I have done that.  And unless you got your ACOG for a song at an estate sale and it smells like cat-pee; do NOT do that!  Mine works great now and the dials work great now, FWIW.  Great scope, though different for sure.  The glass clarity is amazing.

As others say, I learned pretty quick an ACOG is not a scope you should click-adjust for each shot's wind and elevation every outing.  It's very much one you set for that rifle, and don't ever touch it again.

If your plan is to use an ACOG for multiple guns, and they don't share the same zero adjusted back and forth -  well... good luck.  ACOG may not be the right scope for that project.

Yeah he spoke to the listed eye relief but their listed eye relief on this one, and a few others, are pretty far off the mark from reality.

Mine are often a pain in the ass to zero regardless. Not that it’s a terribly relevant complaint either way. When it’s zeroed it’s zeroed. It’s not like anyone is dialing shots with the ACOG or anything.

I think this is going to need to go back to trijicon for service. I had it zeroed spot on, hit a bullseye at 100 yards, then hit another bullseye second shot, then the next shot was way right and high, the next shot after that was way left and high. Then once again I couldn't get back on paper. I know it's not me. This acog just doesn't want to hold zero, idk wtf happened to it because it was just fine on my other rifle. I used to ring steel with it all day at 300 yards and it never lost it's zero before.

In the real world away from the internet everything can and will fail. This includes ACOGs, Aimpoints, EOTechs, and every other optic.

Some things just break more often and quickly than others.

Contact Trijicon and you’ll have a RMA by the end of the day.

Its a common thing for military users, passed down through the decades by experienced TLs. I’ve always assumed it was due to worn out and abused sights. Today I learned that it’s in the manual.

On tracking, I don’t think my VCOG even tracks well, though I haven’t tested specifically for that, because I don’t care with that optic (or an ACOG).

About switching between guns, sometimes that kinda works, sometimes it clearly doesn’t. I lap my receivers in an attempt to increase the likelihood that I can test various optics on a new gun with the existing zeroes being close enough for that purpose. The advice on going back to 25 and zeroing 2.5” low should get you on paper at 100. If not, try at 50 with the 200m hash.

ACOGs do not break