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Discussion Starter · #1 ·
Hey all, I recently got a Magpul MBUS Pro LR rear sight for my 20" FN-A4 AR. There is no clear instruction on how to zero the sight, so I set a 50-200m zero with the small aperture using M855 (rear elevation set to 2). When switching to the large aperture, I'm hitting approx 9" high at 100m and I can't figure out why. I was told by a former marine friend that I should use a 25-300m zero with the elevation set at 3 so that the large aperture was more or less on at 100m, turns out it was off paper but I couldn't tell if it was high or low because I didn't have a spotter. Can someone explain why I'm having this issue? My groups are good, 3-5" at 200m from a prone rest, the barrel is not touching anything when I'm shooting.
 

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Mr. Awesome
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Where, in the circle(on the rear sight) are you putting the tip of the front sight post? It is supposed to be in the center of the circle. Your issue sounds like you have put the sight post at the top or bottom of the circle and when you switch from long range to short range, it’s changing the sight alignment.


And zeroing:


From the article:
951718
 

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Discussion Starter · #4 ·
Proper AR iron sight picture, with either circle size.

View attachment 951719
I'm not new to irons, I place the center of the post to the center of the target, regardless of aperture. I went back and checked last night at 50yds on steel; center hold with small aperture is dead on, large aperture is 9" high I have to aim below the base of the plate to get a center hit. Regular polymer MBUS sights don't have the same issue.
 

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I'm not new to irons, I place the center of the post to the center of the target, regardless of aperture. I went back and checked last night at 50yds on steel; center hold with small aperture is dead on, large aperture is 9" high I have to aim below the base of the plate to get a center hit. Regular polymer MBUS sights don't have the same issue.

A 50-200m zero, IMO is the best use of 5.56 as the your hold over/unders is less than ~2" out to 250m. That means you don't have to adjust the rear turret to make effective hits out to ~250m. The Marines zero to 36-300y zero with hold over/unders of ~4.5".

Which one is better is subjective but a 50-200m zero has less spread for the given range most "combat" occurs at.

Either system requires the rear turret bottomed out or bottomed out +1 and an "F" marked front sight base to work as intended.

You said you had the MBUS set at "2", is that as low as it will go? If not, bottom out the rear turret and adjust the front sight post to get your elevation dialed in.
 
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Discussion Starter · #6 ·
A 50-200m zero, IMO is the best use of 5.56 as the your hold over/unders is less than ~2" out to 250m. That means you don't have to adjust the rear turret to make effective hits out to ~250m. The Marines zero to 36-300y zero with hold over/unders of ~4.5".

Which one is better is subjective but a 50-200m zero has less spread for the given range most "combat" occurs at.

Either system requires the rear turret bottomed out or bottomed out +1 and an "F" marked front sight base to work as intended.

You said you had the MBUS set at "2", is that as low as it will go? If not, bottom out the rear turret and adjust the front sight post to get your elevation dialed in.
Yes, 2 is the lowest setting on the sight. I have an F marked front sight post as well. No issues making hits out to 200m, maybe 1.5" high at 100m but that's what it says on my DOPE chart for 5.56. The only issue I'm having is the large change in zero from small to large aperture.
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Yes, 2 is the lowest setting on the sight. I have an F marked front sight post as well. No issues making hits out to 200m, maybe 1.5" high at 100m but that's what it says on my DOPE chart for 5.56. The only issue I'm having is the large change in zero from small to large aperture.
View attachment 951728 View attachment 951729
This might be a little fiddly, but you could tape off the large aperture and punch a small hole the same size as the small aperture and see if the results change. If they don't I'd suspect the sight is out of spec. Of course it's completely normal for the groups to open up using the larger aperture as well. 9" seems excessive either way. What sort of ammo are you using?
 

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Discussion Starter · #8 ·
This might be a little fiddly, but you could tape off the large aperture and punch a small hole the same size as the small aperture and see if the results change. If they don't I'd suspect the sight is out of spec. Of course it's completely normal for the groups to open up using the larger aperture as well. 9" seems excessive either way. What sort of ammo are you using?
I've tried all sort of ammunition with the same results, mostly 62gr because of the 1:7 twist:
Winchester M855
Hornady 62gr FMJ
Barnaul .223 62gr FMJ
PMC M855
Federal m193
Hornady M193
PPU M193
Tula .223 55gr FMJ
I'll try that tip with the tape and see if it makes a difference. I will also contact magpul to see what they have to say.
 

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I've tried all sort of ammunition with the same results, mostly 62gr because of the 1:7 twist:
Winchester M855
Hornady 62gr FMJ
Barnaul .223 62gr FMJ
PMC M855
Federal m193
Hornady M193
PPU M193
Tula .223 55gr FMJ
I'll try that tip with the tape and see if it makes a difference. I will also contact magpul to see what they have to say.

Good luck, I'd like to hear the conclusion to the story when it's all said and done.
 

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Hey all, I recently got a Magpul MBUS Pro LR rear sight for my 20" FN-A4 AR. There is no clear instruction on how to zero the sight, so I set a 50-200m zero with the small aperture using M855 (rear elevation set to 2). When switching to the large aperture, I'm hitting approx 9" high at 100m and I can't figure out why. I was told by a former marine friend that I should use a 25-300m zero with the elevation set at 3 so that the large aperture was more or less on at 100m, turns out it was off paper but I couldn't tell if it was high or low because I didn't have a spotter. Can someone explain why I'm having this issue? My groups are good, 3-5" at 200m from a prone rest, the barrel is not touching anything when I'm shooting.
sight it in at 25m on the 300 setting
A 25/300 zero is a bit more realistic for your rifle and the MBUS
 

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Mr. Awesome
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I'm not new to irons, I place the center of the post to the center of the target, regardless of aperture. I went back and checked last night at 50yds on steel; center hold with small aperture is dead on, large aperture is 9" high I have to aim below the base of the plate to get a center hit. Regular polymer MBUS sights don't have the same issue.
Well, sounds like your sights are messed up. I was hoping it was something easy.
 
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Discussion Starter · #12 ·
Well, sounds like your sights are messed up. I was hoping it was something easy.
I reached out to Magpul and the technical support engineer was stumped, said it might be something out of spec and to return it on warranty. Their recommendation is a 50-200m zero with elevation on setting 2 using small aperture which was followed. I will post again when I get the replacement sight. Sucks because that sight wasn't cheap and neither was all the ammo I used to figure it out lol.
 

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I reached out to Magpul and the technical support engineer was stumped, said it might be something out of spec and to return it on warranty. Their recommendation is a 50-200m zero with elevation on setting 2 using small aperture which was followed. I will post again when I get the replacement sight. Sucks because that sight wasn't cheap and neither was all the ammo I used to figure it out lol.

I once blew over a hundred rounds of 9mm trying to diagnose a red dot sight issue. Turns out the threaded barrel protector was loose inside the flash hider on my Scorpion.

I though I was just losing it.

Hopefully the new sight works out.
 

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Mr. Awesome
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I reached out to Magpul and the technical support engineer was stumped, said it might be something out of spec and to return it on warranty. Their recommendation is a 50-200m zero with elevation on setting 2 using small aperture which was followed. I will post again when I get the replacement sight. Sucks because that sight wasn't cheap and neither was all the ammo I used to figure it out lol.
Kind of an insult to injury. Hopefully, they get a good set to you. From what I‘ve read, they are a good set (when built correctly). You might bring up the ammo used, if they do find it out of spec. Some 30 round mags might alleviate some of your pain.

I once blew over a hundred rounds of 9mm trying to diagnose a red dot sight issue. Turns out the threaded barrel protector was loose inside the flash hider on my Scorpion.

I though I was just losing it.

Hopefully the new sight works out.
Yeah…..nothing like that ever happened to me. Never.
 
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Kind of an insult to injury. Hopefully, they get a good set to you. From what I‘ve read, they are a good set (when built correctly). You might bring up the ammo used, if they do find it out of spec. Some 30 round mags might alleviate some of your pain.



Yeah…..nothing like that ever happened to me. Never.

Uh huh :cool:
 
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I wouldn’t worry about it. I always considered the larger aperture to be nothing more than a combat sight for indoors, short distances, and low light conditions.

At 100 yards with the large peep on my A2 rifles I never got anything really resembling a group anyway. Just minute of man accuracy. I think your sights are fine. Use the small peep when accuracy is needed and the large peep for the three situations I listed above. I never expected too much from the large peep on my ARs. The small peep usually gives me 4 moa with standard cheap 55g plinking ammo. With the large peep it’s usually 9 or 10. Hard to call that a zero. About the best you can do is draw a circle around all the rounds and mark the center.

I had a .30-30 with the XS ghost ring sights that was a deer hammer in thick woods at 40-50 yards, but on paper at 100 it was hard to get anything resembling a group. It’s the same for the Mossberg ghost ring sights on their shotguns. They just aren’t intended for precision at long range. They are intended for quick target acquisition at shorter ranges and low light conditions when the small peep doesn’t let enough light in.

I don’t think there is anything wrong with the sight. If the small peep is accurate and the large peep hits a man sized target at 50 yards I’d be happy.
 

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Some sight sets will have “same plane” rear apertures where the center of the peep and ghost ring line up at the same distance over the center of the bore so the zero will be the same. Others do not, meaning the ghost ring and peep sight will have different zeroes. If you have a non same plane rear aperture you will have to decide whether you’re primarily using the flip ups for precision practice or as backups in a HD situation and then zero with the ring you’re going to use most often.

If your groups are good and centered between the two and you’re 100% confident you’re holding in the center of the ghost ring (which is harder to line up than the peep) then I would think that is not a same plane rear sight. If they are advertised as such this is a defect of your unit. It is out of spec. If they are not advertised as such, it is not. Measuring center of bore to center of each aperture should settle it. I suspect some rear sights that are marketed as same plane are not consistently so but most people never thoroughly test and measure so manufacturers get away with it.

Most Average owners will zero the small hole and never try to get precise enough with the big one to be sure, or get a rough “minute of man” zero with just the large hole and call it a day and work with the optic.
 
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