New Magpul MBUIS Hitting Way High

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Ben86

Ben86

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I recently installed a flip up MBUIS on top of my Bushmaster AR (without a riser) and after trying to sight it in today it is still hitting so high at 100 yards I can't even stay on paper, and my front sight has run out of adjustment range. My front sight is up so high (to lower the point of impact) it's about to fall out. The MBUIS I have is only windage adjustable. The front sight is the stock sight/gas block that came with the rifle. I've got the windage right on, but the elevation is hopelessly high.

Am I doing something wrong? Is there something I could be missing? Perhaps this rear sight is just not compatible and I need to get one that is also elevation adjustable. I'd appreciate any help/speculation.
 
do you have a straightedge that's long enough to reach from the back of the upper to the front of the gasblock?

if so, run the straightedge along the recoever and measure the distance from the top of the reciever rail to to the gasblock rail.

the magpul sights are designed to mount on one rail, or two rails at the same height.
 
Bushmaster doesn't install the correct front base on their rifles. I honestly don't understand why they even put a front sight base on their flat top uppers, since as it is delivered, the front sight is worse than useless.
 
Just to clarify, it's the front sight base that should be F marked when you have a flat top upper. The front sight post is standard.

And yes, Shrubmaster is screwing you up.

BSW
 
Correct, base vs post. The point is that Bushy screws you and doesn't even bother installing the correct post to make up for the wrong base, so in reality they come with both the wrong base and the wrong post. The end result is that the front sight is worse than useless as delivered with a flat-top upper.
 
My rifle started out as a plain A1 type with a fixed carry handle rear sight. I added a Yankee Hill flat top receiver so I can join the modern world with a red dot sight. Now I'm trying to get some back up sights working with it as well. I don't see a letter designation on the front sight, but since it was an A1 set up I'm sure it must be the shorter type. Thanks for pointing that out, I didn't even know that. I'll have to order that front sight and see if that fixes it. I'm sure a taller front sight is what I need.

I doubt I'll buy another Bushy. When I tried to have the flat top receiver put on the barrel was locked so tight my LGS couldn't get it off. I had to send it back to have it put on. I also don't like the fact that the flash suppressor is welded on. Add this to the list of reasons my next AR purchase will not be a Bushmaster.
 
I'm sure it's 14.5, but how is it other ARs can have customized flash suppressors and sound suppressors installed? Do only mid and full length rifles come with removable flash suppressors?
 
Ben86 said:
I'm sure it's 14.5, but how is it other ARs can have customized flash suppressors and sound suppressors installed? Do only mid and full length rifles come with removable flash suppressors?
Click to expand...


In order for a 14.5" barreled M4 clone to be legal the muzzle device must be permanently attached (welded and pinned) and extend the total length to over 16".



Any rifle with a 16" or greater barrel can be threaded normally.
 
Last edited:
Have you tried the 50m/200m zero? 100 yard zeros and ARs don't mix well.
 
In order for a 14.5" barreled M4 clone to be legal the muzzle device must be permanently attached (welded and pinned) and extend the total length to over 16".

Any rifle with a 16" or greater barrel can be threaded normally.
Click to expand...

So if I want to put a suppressor on it I need to buy a barrel of at least 16"?

*grumble* capacitive touchscreens still have a way to go.
Click to expand...

No kidding, it's like I have 10 thumbs when using one.

Have you tried the 50m/200m zero? 100 yard zeros and ARs don't mix well.
Click to expand...

It's funny you ask. At 50 yards it worked fine. What are the benefits of a 50/200 yard zero?
 
Wait a second.

Are you telling me that Bushmaster AR15s that are flat top, dont have flat top marked FSBs making it nearly impossible to shoot the thing with a standard flip up BUIS?

Thats...... thats quality right there.

Sorry for my ignorance, I dont know this because I buy Daniel Defense and Noveske AR15s.

Which oddly enough, can be sighted in. Who would have thunk it!
 
helotaxi said:
Correct, base vs post. The point is that Bushy screws you and doesn't even bother installing the correct post to make up for the wrong base, so in reality they come with both the wrong base and the wrong post. The end result is that the front sight is worse than useless as delivered with a flat-top upper.
Click to expand...
The “F” marked front sight base is the exact same sight base as the non marked, the only difference is the shelf is .040 inches higher as it simply is not machined down the extra .040. With the extended sight post (+.040) on a standard sight base you get the exact same sight picture as with the “F” marked sight base and a standard sight post.

front-sight-bases.jpg
front-sight-posts.jpg

The non F marked Bushmaster FSB matches the height of the Bushmaster detachable carry handle. If you change the height of the rear sight you may need the taller post.

In most cases you can raise the standard post high enough to sight the weapon. I've never had a issue when using Troy, Matech or LMT rear sights. The base of the post may end up higher than the base of the FSB.
 
For more info on 50/200 zero google ar15zeroing.com. It is a very informative and short read. This is the zero i use for all 3 of my ARs. This zero puts you a couple of inches high or low out to 300 yards.

The above FSB info is spot on. Get the taller sight post and you good.
 
Your bullet will cross the line of sight at 50 on the way up and 200 on the way down. It will be a lil high at 100 and a lil low at 300.

Or do the 25/300

Either way you will have to fine tune it aat the longer distance, but it will get you close.



http://m4carbine.net/showthread.php?t=65679
 
So if I want to put a suppressor on it I need to buy a barrel of at least 16"?
Click to expand...
You could have a gunsmith build you a rifle with a flash suppressor on a 14.5" barrel as long as it is permanently attached and the overall length of the barrel (including flash suppressor) is at least 16" to the back of the chamber (measured with a rod down the barrel touching the breechface). If you are starting with a barrel that already contains a welded-on flash suppressor, it's not going to come off, but you can order a new barrel with permanently attached FS if you want something different.

It is a serious Federal crime (10-year Federal felony, National Firearms Act of 1934) to possess a rifle with a barrel shorter than 16" unless you first obtain a BATFE form/tax stamp, and there's a vetting process involved.
 
Your bullet will cross the line of sight at 50 on the way up and 200 on the way down. It will be a lil high at 100 and a lil low at 300.
Click to expand...

It looks like there will be less deviation in elevation if I use the 50/200 yd scheme compared to a 100 yd zero, because 100 yards is at its highpoint in trajectory. It will also be a lot easier to manage. I plan to try that out as soon as my taller front sight comes in the mail.
 
I've got the new front sight post installed and it works like a charm. I've got it sighted in at 50 yards just fine. Thanks for the help. I've also got a proper front sight tool on the way, a small punch and non-marring pliers don't work that great. I couldn't get the bullet adjustment method to work either. ;)

It is a serious Federal crime (10-year Federal felony, National Firearms Act of 1934) to possess a rifle with a barrel shorter than 16" unless you first obtain a BATFE form/tax stamp, and there's a vetting process involved.
Click to expand...

That's why I'll probably just get the mid length rifle next time around.
 
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