Author Topic: Receiver and barrel alignment causing elevation sight issues  (Read 1889 times)

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Offline Rooftop Korean

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Hello everyone, 
Wondering if anyone out there has a similar issue as myself.  I did a search but haven't found anything on this forum.  When I first shot my Scorpion I found while using the iron sights that it shot very low.  Point of aim and point of impact was about 12" low at 50'.  I tried to adjust the front sight post to the bottom stops but my point of impact was still about 10" low (If I remember correctly).  So I decided to sight in my red dot sight which needed a considerable amount of elevation adjustment. 

So at home I decided to have a look at my HB Industries hand guard and this is what I found.  I laid a ruler against the receiver and found that the end of the hand guard at the muzzle end was about 1/16" low.  Then I looked through the Mlok slots and it looks like the barrel is lower than the rail.







Eyeballing the barrel and hand guard isn't the most accurate way to check barrel alignment but I think there may be a slight issue here.  With the combination of the hand guard and barrel's slight misalignment I estimate that there may be up to a 1/8" muzzle deviation from the receiver's picatinny rail.   Has anyone else experienced this and is there a way to fix this?  Or is it just my mind playing tricks on me?


Offline gtmtnbiker98

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #1 on: July 13, 2019, 10:45:43 AM »
Using the MBUS for 9mm will cause you to bottom out the elevation, since they are designed for 5.56.  I ran out of elevation when I used them on my MPX.  I stick with CZ factory sights on my Scorpions.

Offline Rooftop Korean

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #2 on: July 13, 2019, 11:50:33 AM »
Same result with the stock CZ factory sights. I got the MBUS sight s because I’ll be SBRing this piece with an ACR stock. Can’t use the CZ sights with the ACR stock because they’re way too low.  Maybe I should just run it without any kind of irons or get a new hand guard that’s more in line with the receiver.

Offline Rooftop Korean

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #3 on: July 13, 2019, 12:09:02 PM »
Well, so far after some more research I saw a few threads on a AR15 forum and one here.  Looks like some people had some barrel / Trunnion that were installed incorrectly.  Some look like the barrel wasn’t up to par.  I will do some other investigating and see what I find.  Hopefully my experience will help someone down the road.

Offline OpusXJ

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #4 on: July 13, 2019, 12:23:54 PM »
FYI they do this with the factory handguard as well. The receivers on these guns all vary greatly as to which way they are banana'd.

You should be trying to zero at 25yds~75ft, as 50 is a little close.

Offline Rooftop Korean

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #5 on: July 13, 2019, 12:59:27 PM »
I agree that 50’ is a bit close, but that’s the limit to my local indoor range.  I still think a 10” point of aim / point of impact is pretty significant though when it comes to the front post iron sight at it’s absolute limit.  It shouldn’t be that bad.

I plan on stripping down my Scorpion down to the receiver / barrel assembly and check alignment.  Then most likely completely disassemble and check if there’s anything I can do to realign the trunnion to the receiver.  I hope it’s something as simple as uneven torques applied to the receiver halves.  If not then hopefully shims may be the answer if that’s even a possibility.  Then there’s bore / thread concentricity when I pick up my 9mm suppressor later on down the line.

Offline kb377

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #6 on: July 13, 2019, 01:48:29 PM »
I noticed you have the Sapper handguard...

Mine was zero'ed from the factory and only needed some tweaking at 25 yds.

I installed the Sapper handguard a week or so ago and finally got out to the range this morning...I bottomed the front sight out and it's still about 4" low at 25 yds.

I haven't laid a straight edge across the receiver/handguard yet...I'm curious to see the results.

Offline OpusXJ

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #7 on: July 13, 2019, 02:01:57 PM »
I agree that 50’ is a bit close, but that’s the limit to my local indoor range.  I still think a 10” point of aim / point of impact is pretty significant though when it comes to the front post iron sight at it’s absolute limit.  It shouldn’t be that bad.

I plan on stripping down my Scorpion down to the receiver / barrel assembly and check alignment.  Then most likely completely disassemble and check if there’s anything I can do to realign the trunnion to the receiver.  I hope it’s something as simple as uneven torques applied to the receiver halves.  If not then hopefully shims may be the answer if that’s even a possibility.  Then there’s bore / thread concentricity when I pick up my 9mm suppressor later on down the line.

There isn't really anything you can change in regards to alignment. The way the receiver clamps the trunnion can't be changed. What do you plan on re-torqueing the receiver bolts to?

If you remove the handguard and put just the gun on a flat stone you'll probably notice it bows in the middle. With the factory handguard on it will likely look like the Sapper.

Offline John A.

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #8 on: July 13, 2019, 02:23:43 PM »
It's not the gun.  It's physics.

Flip up AR15 sights were designed for AR's, whose front post sit about 2-1/2" above the center of the bore.  Since they sit up so much higher, what that does is push the poa/poi a lot lower on guns whose barrels do not sit the same distance because they were not designed to have that much adjustment.

If you can't adjust the front sight enough to zero it with the front post bottomed all the way out and you're still shooting low, you'll either need to start filing the top of the front post off, or switch to a more appropriate iron sight.



When people ignorant of guns make gun laws, you end up with ignorant gun laws.

Offline kb377

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #9 on: July 13, 2019, 02:51:40 PM »
It's not the gun.  It's physics.

Flip up AR15 sights were designed for AR's, whose front post sit about 2-1/2" above the center of the bore.  Since they sit up so much higher, what that does is push the poa/poi a lot lower on guns whose barrels do not sit the same distance because they were not designed to have that much adjustment.

If you can't adjust the front sight enough to zero it with the front post bottomed all the way out and you're still shooting low, you'll either need to start filing the top of the front post off, or switch to a more appropriate iron sight.

I was having the same issue using the original CZ sights.

 Just went and checked my Sapper handguard...




Between this and having to file the bottom M-Lok slots because they weren't in spec (too narrow), looks like this was a $50 hard lesson.
« Last Edit: July 13, 2019, 03:14:12 PM by kb377 »

Offline Rooftop Korean

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #10 on: July 13, 2019, 05:53:34 PM »
I’m getting the same results with the stock CZ iron sights and the Magpul Mbus Pro sights.  With the CZ stock iron sights and the Magpul Mbus sights I’m getting a 10” a 12” difference between point of aim / point of impact.  Both are yielding the same results with the front sight post at the lowest mechanical setting.  I don’t think it’s the iron sights. 
« Last Edit: July 13, 2019, 10:30:37 PM by Rooftop Korean, Reason: After thought »

Offline Rooftop Korean

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #11 on: July 13, 2019, 05:59:26 PM »
There isn't really anything you can change in regards to alignment. The way the receiver clamps the trunnion can't be changed. What do you plan on re-torqueing the receiver bolts to?

If you remove the handguard and put just the gun on a flat stone you'll probably notice it bows in the middle. With the factory handguard on it will likely look like the Sapper.

I plan on re-torque to 15” pounds on all received screws. I heard factory specs were 18” pounds.  I’ll lay it out on a flat surface to see which way my Scorpion is biased towards. 

Offline Rooftop Korean

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #12 on: July 13, 2019, 10:07:32 PM »
Ok, so this is what I found so far.  I disassembled my Scorpion down to the Barrel and Receiver assembly and took some measurments by placing a straight edge on the top of the receiver and checking the differences between the muzzle end and chamber end with a square.  My measurements indicated that the muzzle end is lower by 3/64" using my non scientific method which doesn't really say much.  So I took the halves apart and found that the fit between the Trunion and the receiver halves are very close.  Nothing can really be done without taking a big risk and turning it into a expensive paper weight.  I know the left halves are available for purchase and the right half is not.  So I reassembled the receiver using 15 inch pounds with loctite on each screw and put it all back together with the exception of the hand guard and the left side sling loop.  I got tired of scraping my hand on it when operating the charging handle.  If anything I'll install a Magpul Mlok QD on the handguard.

What I did with the HB Industries hand guard was sand down the one end at an angle to level out the hand guard with the receiver's picatinny rail.  Started with #80 grit to get it roughed in then test fit it to the receiver till it worked out.  Then I stepped it all the way up to #1200 grit. 



It didn't take much to shape the end of the hand guard, and polishing it off with the finer grits was pretty quick work.  If you decide to try this out, proceed with caution and at your own risk!  Then again it's a $50 part.  I wanted to save this part because I think it just looks so cool and I agree with others out there making the Scorpion look like a HK UMP.

Here's the end result.  I decided to move the C-more sight to the back because it kept on interfering with my HK slap technique.  Something I've been perfecting for may years with airsoft guns.  The irons are there purely for backup and the placement is similar to what some people do with their handguns utilizing a RMR and irons.  I'll be taking Anya to the range tomorrow with hopes that correcting the handguard will fix my sight alignment problem and that my sight configuration will be feasible. 



Wish me luck!
« Last Edit: July 13, 2019, 10:32:02 PM by Rooftop Korean »

Offline st381183

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #13 on: July 14, 2019, 12:40:57 AM »
Has anyone contacted Pakse or HB Industries to see if they are aware of the alignment issue with the Sapper Handguard?  I put one one last week but haven’t been to the range with it yet. My eyeball micrometer accurate to +/- 1” doesn’t notice any alignment issue on the top rail between the receiver and the handguard but that doesn’t mean there isn’t an alignment issue either.  I left my red dot on the receiver during the change out and the dot was about 4 clicks to the right of the front sight post but the elevation was good to go. 

Offline Rooftop Korean

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Re: Receiver and barrel alignment causing elevation sight issues
« Reply #14 on: July 14, 2019, 12:56:14 AM »
I haven't contacted HB Industries yet.  I like to have all my ducks in a row before I contact a manufacturer / company.  Me, I had to adjust my C-More and Holosight in elevation quite a bit.  Windage just needed some minor adjustments. 

When I test fire it tomorrow I'll send them my findings.  I hear nothing but good things about HBI, and I like to make a awesome product better if I can.