Iron Sight Issues on my MK12 Mod 0 upper

[Tengo1]

So I bought a custom built (meaning someones build upper) on AR15.com last week. Its Build Specs are this:

White Oak upper (with key hole mark)
White Oak semi auto BCG
White Oak SPR Barrel 18 inch barrel
PRI Gen 3 forearm
PRI flip up front sight/gas block
PRI SPR PEQ top rail
ARMS #40 Rear sight
Ops Inc. 12th model collar and brake

It will be a few weeks before I will get a scope and rings for it, but I had the itch to go shoot it, and thought what the hell, I'll go shoot with the iron sights and get them sighted it. After all, half the benefit of having the ARMS lever scope rings is being able to snap the scope off in a pinch and use the irons.

I got out to the range today and first things first I made sure everything was tight on the upper, namely the rear sight. I put out a big cardboard board at about 30 yards to get me close to zero, then planned on moving it out to 100 yards to refine it. I was using a Hopps front benchrest and a bean filled rear bag. I fired off 5 rounds and immediatley saw that I was shooting clear to the left, like 14 inches to the left. And about 3 inches high.

I decided to focus on the windage first, as I probably wouldn't touch the elevation tell I moved out to 100 yards. I cranked the windage on the ARMS rear sight to the right several clicks and shot again, still clear to the left. Cranked it to the right agian, I was closer, but still to the left. I finally moved it all the way to the right tell the adjustment knob would no longer move, aimed at a new target and shot 4 rounds, this is what I got:

This is a view of the rear sight as is:

I'm still a solid 3 inches to the left! And I cannot adjust the rear sight any more to the right, If I adjust the opposite way, so the rear sight is say, lined up with the middle line on the back of the sight, I am a full 13-14 inches off to the left, as in completely off the paper. I am so frustrated, I have never had an issue sighting in an AR before. I dont understand what my issue is, Canted front sight/gas block maybe? I gave the front sight a good hard look over, and it appears to be to centered to me, but maybe its off just a little, I dont know. To me the front sight is the only thing that could explain not being able to adjust any more to the right and still be this much off.

I also realize once I get a scope on this upper I may hardly if ever actually use the iron sights, but if I had to pull off my scope and use my irons, I would like to know that the sights where zeroed.

On a side note. I noticed when cleaning the inside of the upper that as soon as I touched the gas tube with a q-tip it rattled. It has very little movement but it does move. Every time I tapped it with a q-tip it makes a "tink tink tink" sound. I have two other AR's and both of their gas tubes are solid, zero movement, zero bounce, sound etc. when you touch them. Granted they both have standard handguards with the standard delta ring, etc. And this upper is free floated with the PRI Gen 3 handguard, so I dont know if its just a matter of the handguard, but it is something I noticed. What say the hide on both these issues.

 

[aj_brown]

Re: Iron Sight Issues on my MK12 Mod 0 upper

Your gas tube should move if it does not it is in a bind. You need to rotate your front sight to the left as you set behind the rifle.

Alan

 

[Drifter™]

Re: Iron Sight Issues on my MK12 Mod 0 upper

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AJ Brown</div><div class="ubbcode-body">Your gas tube should move if it does not it is in a bind. You need to rotate your front sight to the left as you set behind the rifle.

Alan</div></div>

+1

 

[neal0124]

Re: Iron Sight Issues on my MK12 Mod 0 upper

I have a mk12 mod0 with the same front/rear sight. I left the rear centered and zeroed the rifle with the front sight. Loosen the front cross bolt on the front sight/gas block all the way out. Then loosen the rear cross bolt just enough so you can rotate the block by hand. Move it a tiny amount to the left then tighten it back up and fire off a few rounds. The gas port on the barrel is much smaller than the port on the gas block, so you will have a bit of room to play around with. Zero it with the small aperture.

 

[Tengo1]

Re: Iron Sight Issues on my MK12 Mod 0 upper

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: neal0124</div><div class="ubbcode-body">I have a mk12 mod0 with the same front/rear sight. I left the rear centered and zeroed the rifle with the front sight. Loosen the front cross bolt on the front sight/gas block all the way out. Then loosen the rear cross bolt just enough so you can rotate the block by hand. Move it a tiny amount to the left then tighten it back up and fire off a few rounds. The gas port on the barrel is much smaller than the port on the gas block, so you will have a bit of room to play around with. Zero it with the small aperture. </div></div>

I followed your instructions step by step and it worked perfectly! Thank you so much! This is what I love about sites like this. And of course, complimentry pics.

 

[NRAShooter]

Re: Iron Sight Issues on my MK12 Mod 0 upper

If your front sight bridge is held in place with set screws you can adjust your windage by moving your front sight. Use a Laser Bore Sighter and it will save you some rounds.

If your front sight bridge is held in place by pins, you need to disassemble the rifle and check to see if the barrel was over torqued upon assembly. There's the possibility the barrel had just a little too much torque and the front sight is not directly inline with the centerline of the bore.

Either way, call the armorer and its an easy fix.

 

[neal0124]

Re: Iron Sight Issues on my MK12 Mod 0 upper

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tengo1</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: neal0124</div><div class="ubbcode-body">I have a mk12 mod0 with the same front/rear sight. I left the rear centered and zeroed the rifle with the front sight. Loosen the front cross bolt on the front sight/gas block all the way out. Then loosen the rear cross bolt just enough so you can rotate the block by hand. Move it a tiny amount to the left then tighten it back up and fire off a few rounds. The gas port on the barrel is much smaller than the port on the gas block, so you will have a bit of room to play around with. Zero it with the small aperture. </div></div>

I followed your instructions step by step and it worked perfectly! Thank you so much! This is what I love about sites like this. And of course, complimentry pics.

</div></div>

Put a scope on that thing!

Now that I figured out how to post pics I have to show off mine. The little one and the big one. Both of them have a PRI flip up front sight/gas block and that's how I zeroed them without adjusting the rear sight more than a couple clicks.

 

[regatoni5]

Re: Iron Sight Issues on my MK12 Mod 0 upper

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tengo1</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: neal0124</div><div class="ubbcode-body">I have a mk12 mod0 with the same front/rear sight. I left the rear centered and zeroed the rifle with the front sight. Loosen the front cross bolt on the front sight/gas block all the way out. Then loosen the rear cross bolt just enough so you can rotate the block by hand. Move it a tiny amount to the left then tighten it back up and fire off a few rounds. The gas port on the barrel is much smaller than the port on the gas block, so you will have a bit of room to play around with. Zero it with the small aperture. </div></div>

I followed your instructions step by step and it worked perfectly! Thank you so much! This is what I love about sites like this. And of course, complimentry pics.

</div></div>

beautiful stick! what did this end up running you? and what glass are you gonna drop on this bad boy?

 

[Tengo1]

Re: Iron Sight Issues on my MK12 Mod 0 upper

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: regatoni5</div><div class="ubbcode-body"> beautiful stick! what did this end up running you? and what glass are you gonna drop on this bad boy? </div></div>

The upper was $1100 thru a member on this site. It was used with 200 rounds down the tube but appears in excellent condition. The lower is from another gun I have. I plan on eventually getting the correct Leupold Mark 4 scope for it, but with my "gun funds" being a little low right now I am thinking of getting a Vortex Viper PST 2.5X10 scope to put on it for the time being. It wont be a "correct" build becuase of it. But it will be a year or two before I can get the right scope and OPS Inc. 12th model suppressor for it.