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Old January 2, 2020, 12:30 AM   #1
Metal god
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Does this bug you as much as it does me ?



Got this upper the other day for a iron sights only build I wanted . Tried a cheep sight real quick on it the other day ( first time out ) and it needed to be way over to the left to be dead on at 50yds ( I like the 50/200yd zero) . I figured it was the cheep UTG sight so when I got my battle sight in I thought all would be good . WRONG

That pic is 10+ clicks to the left off center which bugs the hell out of me . Eye balling it , it appears to be off center to the flat top which would indicate the front sight base ( F-marked ) is canted ????

I only was able to shoot at 50yards but I'm assuming since the front on rear sight don't line up with the bore . At 300+ and maybe 200 yards my POI is going to be off horizontally and get worse the farther out I shoot ?

This is a dissipatar upper which means mid length gas system with a rifle length handguard and front sight base on a 16" barrel . The front sight appears to be pinned in like a normal one would be ( no set screws ) . I've only removed one front sight like this and reinstalled and it lined back up perfectly with no real effort . At the time I just assumed how the pins engage the slots in the barrel it self aligns ???

My question is , Is that accurate or can I tap the pins back a bit and re-align the front sight to center it up then tap the pins back in ??
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Old January 2, 2020, 01:17 AM   #2
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At first I thought you were talking about the white lines not lining up with the corners of the aperture! (I know, now that's all you will think of!)

Front of the receiver is probably out of square. If you know someone who has an AR lapping rod, just lap the front of the receiver. If you pull the front sight pin and it's tapered, see if you can put it back in from the other side.
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Old January 2, 2020, 02:10 AM   #3
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Interesting, I have a lapping rod . That didn’t even cross my mind as a fix .

Oh I saw the lines but when shooting it does not “seem” to be an issue .
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Old January 2, 2020, 12:54 PM   #4
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I think this is a low quality sight, but lapping never hurts really.
This is a decent tool.


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Old January 2, 2020, 02:24 PM   #5
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It’s a Troy sight and although not top tear , I’ve always considered Troy to be a good quality manufacture . Also the two different sights I’ve tried had the same issue which leans towards the upper being the issue but not guaranteed . I put a standard Troy battle sight on a AR I gave my son years ago and it’s been perfect going on 7 years now .

I forget which lapping tool I have , think it’s a Brownells brand . It’s worked well in the past . I’ll likely give that a try if I can find it haha .
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Old January 2, 2020, 04:00 PM   #6
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Mine is in the same exact position, and it was rock solid hitting man-sized targets at 300 yards. Personally the only time I'd mess with it if there isn't enough adjustment left.

Also, don't look too closely in the mirror and see that one of your eyes is lower than the other. LOL
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Old January 2, 2020, 05:06 PM   #7
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FWIW... I have "shimmed" Pic. rail items before to help them be more centered.

Tin Foil could be used, on one side or the other, dependent on the direction you are trying to shift. Cut away / tear away excess.
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Old January 2, 2020, 05:26 PM   #8
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I'll agree it can be about the upper receiver face,but thinking about the machining operations I'd guess (I don't know for sure) the barrel extension counterbore,barrel nut threads,and receiver face are all cut in the same setup. Likely the BCG bore,too.

Anything could happen,but I'd like to think the receiver face is better than that.

IMO,lapping is about getting it flat,knocking off a high spotso the barrel extension has a stable seat.

I'd be more suspect of indexing. How precisely is the front sight base/gas block indexed to the barrel extension pin that indexes the barrel extension to the upper?

While maybe its not supposed to happen,early on I used the little blue GI armorers booklet to build. I used the GI type armorers vise jaws to clamp on the barrel OD to hold the upper as I torqued the barrel nut.

Which means I was holding the barrel,not the barrel extension.

Murphy's law being what it is,I had a barrel,extension turn.

In fairness,the military process of setting a barrel extension is not the same as setting up a match barrel.

Don't do that with match grade barrels. So I bought the long bar that fits through the upper and engages the locking lug recesses. No more problem.

I think there is a reason why the set screw type NM gas/block/sight base can be adjusted. So you can tune that.

Ideally,the gas port is drilled to be in a groove,rather than cutting a land.

So the setup for pinning the gas block has to consider indexing to the rifling twist.

I don't think they figure being OCD about visually centering the sights into the battle rifle tolerance stack up on all that.

Last edited by HiBC; January 2, 2020 at 05:31 PM.
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Old January 2, 2020, 06:48 PM   #9
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My upper block, mates with the barrel extension and not the receiver too.
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Old January 2, 2020, 10:56 PM   #10
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Nope, doesn't bother me at all.

Now, if it was my rifle....
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Old January 3, 2020, 03:53 PM   #11
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Wouldn't bother me at all either. Unless you plan on going into a cyclone in which you were experiencing a 85 mph crosswind and need all of that adjustment and then ran out of room.
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Old January 3, 2020, 08:37 PM   #12
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I'd be more suspect of indexing. How precisely is the front sight base/gas block indexed to the barrel extension pin that indexes the barrel extension to the upper?
I think this is a good place to start and since the barrel will be off I can lap the receiver ( cus I can ) . I'd like to look at all that this weekend but I recently moved my reloading "room" to a reloading "area" which is much more confined . I've not even set up my vice yet and not sure there is a workable space for it . There's room for the vice but not the extra space needed to hang the upper out from . I'll need to look at all that tonight and see if it will work .

I'll take pics as I go FWTW lol .
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Old January 12, 2020, 05:23 PM   #13
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UPDATE

Well I got my vice mounted . Not a whole lot of room but think it will work



Here are a couple pics of just eyeing it down the upper . The pics don't show much but when holding it the front sight looks canted to the left to me .





Now to pull the barrel , I want to pull the barrel with out removing the front sight so I need to remove the gas tube from the low profile gas block . Thought I'd be able to just loosen up the gas block and slide it forward to the sight but there's not enough room for the tube to clear the upper and barrel nut .
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Old January 12, 2020, 06:06 PM   #14
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If I may ask,Then what? You are kind of stuck with the tapered pins locating the front sight.

You are stuck with the pin in the barrel extension clocking with the slot in the upper. Before that gets monkeed with,remember your bolt lugs need to index into the barrel extension.

Here on TFL I've seen a pic of the receiver slot widened...not a good plan.

Which leaves the barrel being screwed into the barrel extension.

A fancy match barrel might be gently torqued and hi-temp loctited into the barrel extension.The idea is to not stress or distort the chamber.

Different priorities on a GI type battle rifle barrel that may be subjected to dumping a basic load of ammo full auto during a rolling disengage.

My undersyanding is that once the barrel and receiver assy are chambered and headspaced,that index dowel pin in the barrel extension is driven in to crush and stake the barrel threads . The barrel and exptension are locked.

I'm not trying to bum you out,just trying to keep you from opening the worm can.

I don't know of a remedy. I do seem to remember there is a "more than I would like to see" tolerance on the sight leaning one way or the other per GI spec.

If you go to the White Oak site,they describe how they make the barrel and sight tower adjustable,but they use setscrews rather than the tapered pins.

Those tapered dowels make sense on a GI battle rifle,but I just don't buy those barrels.

You have plenty of sight adjustment to handle any wind condition you could hit anything with,anyway.

Its just a mental thing.

You could get an ACOG or other optic.

Last edited by HiBC; January 12, 2020 at 06:14 PM.
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Old January 13, 2020, 11:41 AM   #15
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If I may ask,Then what? You are kind of stuck with the tapered pins locating the front sight.

You are stuck with the pin in the barrel extension clocking with the slot in the upper. Before that gets monkeed with,remember your bolt lugs need to index into the barrel extension.
That is the question I have . I originally asked that very thing . If there is "no" wiggle room with the front sight and how the pins index the front sight . I do believe I am a bit at a stale mate . All my AR's are purpose built and this one was/is to be a SHTF build that is light weight with no optics that can fail and a front sight that is "LOCKED" in place . This was to be a no frills just works when it needs to rifle .

I guess I wont pull it apart just yet . My next step is to go test it at 50 , 1 , 2 & 300yds and see if It's going to shoot more and more right the further out I shoot . 300 is about as far as I can shoot accurately with irons so If I can still hit the target at that distance "maybe" It'll be good enough .

Any other suggestions are welcome
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Old January 13, 2020, 11:53 AM   #16
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Seen this before

If you hold the receiver in a vise block and loosen the barrel nut and put an angle finder on the top of the receiver and then put it on top of the front sight ears you should be able to rotate the barrel assmbly until the angle reading are a close match. Then carefully tighten the barrel nut taking care not to rotate the barrel assembly.
I have found that there is usually a bit of clearance between the pin in the barrel extension and the slot in the upper.
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Old January 13, 2020, 01:46 PM   #17
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You can shoot at those yardages but you can also make an inverted lollipop target, with an aiming point at the bottom of a tall piece of paper, and a vertical line up from the center of it. When you hang it, use a plumb bob to get the line vertical.

Shoot at the aiming point to zero, then add some elevation to the rear sight and shoot another group, and so on. If the groups stay centered on the vertical line it is likely that you have no real problem at all.

On an AR-15, the 'standard' come-ups are 2, 3, and 11 minutes up from 100 yard zero, for 200, 300, and 600 yards - just to get on paper reliably. So if your vertical line is a foot (12 inches / 12 minutes at 100 yards) you can learn quite a bit about potential elevation-induced windage changes.

If you find that you'd really like / need to turn the front sight post as close as you can to vertical, there is an option that will work but I hesitate to bring up because it sounds so crude.

Background: The Bushmaster DCM model sold through the CMP in the middle 2000s was a good, but not superb, rifle for NRA/CMP Service Rifle class. The front sight base was held on with Locktite 290, Sleeve Retainer - no cross pins or set screws at all. Claim was to not to stress the barrel, but I think it was just cheaper than flats and setscrews per White Oak etc.

Edit - I was wrong about 290, the current name is Permatex PX64000 Sleeve Retainer. I do not know the Loctite number, it is marketed under the Permatex brand name at autoparts stores. Label says "High Temperature Sleeve Retainer" and has a picture of cylinder liners being pressed into a block.

This sounds really weak and crappy but that stuff takes being heated with a propane torch, and the sight base being vigorously beaten with a stout plastic mallet, to remove it. I know this sounds full-of-excrement but it is true and verifiable.

You can drive out the cross pins, free up the sight base, turn it to get it where you want it, scribe a line, remove the sight base, clean the barrel and base with acetone, and re-assemble with the 290 to hold it.

I understand, SHTF rifle and all, but it will not come loose without significant heat and force. You will find this out if you do not get it where you want it the first time.

Best of luck and enjoy your problem and its solution. Life is boring if it all works right the first time (my opinion only).
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Old January 13, 2020, 02:48 PM   #18
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Not to be arguementative,but the transverse notch in the barrel to accommodate the tapered pin will likely have effect on the harmonics of a pencil barrel if a tapered pin is not filling the hole with a line to line fit.Its a stress riser.

The stress free practice of Loctite to hold a gas blocking place likely does improve accuracy. The absence of the cross notches to accommodate the pins is probably more significant.

I can think of three theoretical ways to center the rear sight. Ome is a new barrel,without a sight base installed. Yankee Hill makes a steel sight.gas block that clamps on. IMO,even with a 16 in bbl,I'd go with a mid length gas system. No need for a sight base and a lo-pro gas block.

The only other fixes I can think of,I don't recommend. You could offset the rear sight with a vee-block sort of shim to fit between the rail and the sighton the left side. It would require a new clamp to me made for the right side with a dimensional compensation for the shim. Its a routine thing for a toolmaker.

If you have a mill and skills,see what you can do. If you have to pay for it,a barrel is cheaper.

The Wild Man might just figure "Nothing to lose" and grab the oxy-acetylene torch and big crescent wrench. Quick red heat to the sight tower and bend it. I wouldn't do that.myself. The "claimer class" stock car racer just might get it done.. Skilled blacksmithing is a respectable craft.

But,lets look at the geometry of your concern about sight in. The off center appearance of the aperture between the wings is misleading. Its only half as bad as it looks. The actual offset of the sight from center,per my semi-calibrated eyeball,is between .050 and .060 inch. Because you subtract from one gap as you add to the other gap,it looks twice as bad as it s.

Now,lets dismiss spin drift, Coreaiis (sp?) effect,and the wind.We'r just looking at the effect of the sight offset regarding WINDAGE.To keep it simple,for now we will ignore ELEVATION... Lets say your sight is offset 1/16 in.

Revisit the definition of "parallel". Two lines, equidistant apart. They neither converge nor diverge.

So lets have two lines, parallel,offset 1/16. One is line of sight,the other is bullet path. Remember,ignore drop for now.

If the line of sight is 1/16 offset from the precise center of the group,its offset 1/16 in a 50 yds or 500. Parallel.

At 100,or 200,or300 yds,I can't shoot well enough to discern 1/16 in group center offset. In other words,it is a matter of no practical significance.

Do you have,and use,a bubble level on the rail to monitor rifle cant? Probably not.
Do you suppose yu cant the rifle as much as your sight post leans?

No need to split hairs when you are splitting kindling.

Its a grab and go utility rifle.

The barrel assembly is where the problem is. Its like an out of square foundation in concrete. It is what it is.

Update,to be slightly more technically correct ,lets change "two parallel lines"to "two parallel vertical planes" offset 1/16 in. The line of sight is in one plane,the bullet path the other

Last edited by HiBC; January 13, 2020 at 03:31 PM.
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Old January 23, 2020, 04:28 PM   #19
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Canted FSB is a common complaint with the PSA dissipator uppers. They’ll take care of it for you if you contact them, from what I’ve read.
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Old January 28, 2020, 03:19 AM   #20
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Metal god,

My answer to your question is, yes.

Especially when you are excited about an iron sight only build.



I know your pain and I am sorry.


Good luck...
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Old January 29, 2020, 08:36 AM   #21
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I had a AR that was canted like that and it really pissed me off. I eventually removed the front sight then installed a gas block that was large enough to cover the holes in the barrel and had a rail on top for a flip-up front sight. Driving out the pins in the front sight seemed impossible until I purchased action block from Brownells. That held the barrel in place while knocking out the pins.
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Old January 29, 2020, 04:01 PM   #22
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The Yankee Hill Machine clamp on gas block with front sight is a nice unit and would solve the problem.
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Old January 29, 2020, 06:10 PM   #23
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I haven’t had a chance to go shoot it out to 300yds yet . I’ll zero at 50 yards Then shoot it at 100 200 300 yards and see if the point of impact drifts. If it does significantly then I will have to address this issue .

Thanks for all the help and suggestions so far I’m considering several of them depending on the total issue
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Old February 28, 2020, 09:14 PM   #24
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UPDATE

I finally got out and shot out to 300yds today .

If my front sight is canted to the left , which way would my POI shift the further I shoot ?

I sighted the gun in at 50yds then shot at 100yds and it seemed fine there . I went to shoot at 200yds but I did not use contrasting targets with the cardboard and could not see the individual targets . I then shot a 5 shot group at 300yds which had 3 a tad high (4") of POA but dead center and the other two way high and left ( 10"h 8"L ) . That didn't seem to bad but I later shot a 10 shot group at 300 and all were way high and left ( 12'h 8"L )

There was some wind out there later in the day when I shot that second group though .

I really needed to test more but I was testing three other firearms as well and there was only so much time to get each one set up correctly with this AR being the least important of the ones I brought .
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Old February 29, 2020, 12:27 PM   #25
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Rear sight movement moves the bullet impact in the same direction, front sight movements are inverse
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