M*CARBO Brotherhood

SUB2000 Optic Mount Issues

SO! Frustrated and disappointed I walked back into the house from the short range on the property because I just cannot get my Sub2000 2nd gen to Zero. I purchased the mount and rear sights back in Sept 2019. I immediately installed and zeroed the bushnell red dot. Ran like a sewing machine on some crazy old ladies desk. Smashing steal, FAST & CHEAP just how I love it.

Fast forward to first part of March and I wanted to try to put the Vortex Viper on the mount. It would absolutely not zero. All over the paper. I thought to myself, maybe I’m just having an off day? Happens to us all right? Put it back in the cabinet and left it alone for a few days. Pulled it back out and SAME PROBLEM. I took the Viper off and now I can’t get the Bushnell that was originally on the rifle to zero either.

I mounted both optics to a standard AR, bore-sighted, and BAM zeroed in 6 rounds EACH. So I ruled out several things. One, the optics. I ran off probably 25 round each on a different rifle and they both held zero on a much more powerful rifle. I’m at a loss.

There’s only two other possible options. One, the MCARBO mount. Two, the rifle. I’m stumped and it’s been a long time since I’ve been stumped. Let me preface this by saying “it’s not me”. I’m prior active duty army, competition shooter, current Sheriff’s Deputy, SWAT/ K9. I eat, breath, and sh!t shooting. I also asked several other competent shooters to try it. NADDA.

Has anyone else had such an issue. I’m talking bullets walking 12 or more inches at 25 yards?

Any, and I mean absolutely ANY ideas and thoughts are welcome here. I love this rifle and I REALLY want it to work.

I don’t have the optic mount but any rifle that starts to shoot erratically, I look at the crown of the barrel first.

Second I would suspect something loose on the optic mount since it is not a standard mount, it has movement.

Try taking your optic and mount it strait to the gun rail not using the optic mount. That will shoe you if it’s the rifle or mount.


12 inches at 25 yards is insane. I’m no where near the competent shooter you are, and how loose does that mount have to be to throw it THAT much. I would be checking out that polymer on the fore end real good. Maybe your starting to develop a crack in the locking point? Worth checking out as it has happened to a few of the brothers on this site. Good luck with it tho, hope you get it sorted out soon.

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Thanks a good idea. Completely remove the mount from the equation and see if I can get it to zero that way. Thanks for the suggestion!


Thank you very much! I appreciate the advice. I’m going to go through everything to do with locking down the fore end with a fine tooth comb. I can get close to a zero and it starts to walk hard right nearly 12 or so inches off of the paper then erratic and everywhere after that.


Got the same issue!! Just came back from the range impossible to zero it !! So piss right now
Brand new mcarbo optic mount and brand new sig romeo5!! I notice some play up and down on my optic mount where the romeo5 sitting is this normal on the handguard everything is snug ???
Thanks for the help

I’m honestly not sure of the issue. I removed the mcarbo mount and it seemed to zero a lot better. Try mounting the romeo directly to the rifle and see if it will zero that way.

I tried a burris optic and a vortex optic and it would not zero on a bench rest/vise with the mcarbo mount. I mounted the optic directly to the rifle and the zero was not great but it zeroed.

Nothing appears to be wrong with the mcarbo mount. I see no abnormal play. The only issue I see is the rubber bushing sleeve the mount rests on when open or flipped up is a little chewed up.

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@deancoffman & @DonRozon

  1. the springiness is a design feature. spring pressure holds it consistently as it is pressed against the latch. this does mean pressing on the optic while lining up a shot will open up groups.

  2. you are attached to a polymer forend. So shooting positions that load it up (e.g. sling stabilizing) will absolutely move POI.

  3. That polymer forend is fixed by a thin square of metal near the barrel nut. Is the forend secure? Can you wiggle it relative to the barrel?

  4. Torque? Hopefully, you were gentle torquing the mount to the forend. Polymer is VERY easy to deform. The skinny aluminum mount legs need to nestle securely onto the polymer without digging it out.

Finally, I don’t have experience with the Sig optics and their mounts. My guess is that they use an undersized stabilizing crossbar so they can also fit to Weaver rails. Picatinny should be square that fits Picatinny gaps precisely. If it is a skinny round bar, be certain to gently press toward the muzzle while securing the optic to the MCARBO rail to keep recoil from sliding it back and forth.

Best of luck hunting the problem. These are clues I’d investigate.


Thanks a lot Dred
I will give you updates this week


mine has that slop too. i’m not sure if it’s supposed to do that. I haven’t had a chance to try and zero it yet.

Back during the testing phase, I overloaded the hell out of the Mcarbo mount. once zeroed it always came back to zero. all the way up to a ACOG on a aimpoint on flipped GG&G cantiliver mount AR series from Burris… But on the universal lockdown so you can go either weaver or Picatinny , Ive had issues with those before on movement. my breakin dot is a inexpensive truglow 20 mm. square lug. locks in everytime.
Just went thru this on a PS90. sighted in with trueglow. couldnt stay constant with a Bushnell dot. as in locked in a vise. even with the 5.7 cal it was bouncing the mount. I machined a square shim that slid over the “universal” round pin, and the issue went away.
took a lil fitment.
the other is forend movement. they ways around that to…
keep us posted…


I found that the mcarbo mount an a Sig romeo on sale on Amazon for 149.00 each is the cats …an is zeroed out of the box and I’m snake bit! Nothing works as planned but this setup works nail driver at or under 100 yards very impressed with the new sig red dot Romeo sight an M-Carbo flip mount,very sturdy an always zeros!


It’s a hard to beat combo for sure

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Have you looked at how your upper/barrel hand guard assembly locks up to your lower? It’s not exactly a robust system. IMO that’s one of the issues I see with the MCarbo mount, it’s on the lower instead of the upper/barrel.

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One thing that I’ve found out about working on various plastic parted firearms is that when tightening up accessories or bolts/screws on the weapon itself , the tightening process should be done in sequence and evenly. And, use blue LockTite on all screws and bolt threads.

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This👆. Exactly my though. Make sure the optics are good on the gun, not the mount. Go from there.