First off, I am new to join although I have been visiting this site for some time now and it is far and above any other for quality content and ADULTS. I just recieved a new 14.5" midlength upper from BCM. I coupled that with a S&W M&P lower with a DD parts kit. I put on a Magpul flip up rear sight. When I took it out to zero my rear sight needed to be adjusted almost all the way to the left to get on target. I e-mailed BCM and they blew it off as if it was a non issue. My question is, should I be at all concerned about this? It seems it will be an issue for co-witness if nothing else. Is there some fix for this? Any imput would be greatly appreciated. By the way as you may have assumed I am relatively new to the AR platform. I have owned only Ohio legal hunting weapons until I purchased a DPMS sportical this past spring (laugh here!). However after research on this site I moved up. I think.
Have you tried letting someone with some time on the AR-15 shoot it?
You could have a canted front sight, but it could also be a number of other things as well. I always find it helpful when your having to really crank the rear to one side to have someone else shoot the rifle to eliminate user error.
Eta: where were the bullet impacting when you first had it setto a mechanical zero?
x2 on letting another set of eyes give it a go
Yes, my brother in law shot it as well. He however, is less experienced with ar’s than I am. We both have plenty of experience shooting other weapons i.e. shotguns, muzzle loaders, bolt action rifles. To get zero on the BCM the rear sight had to be set a couple clicks left of the second to the last hash mark. I didn’t know if it could be the Magpul or the Fsb or if it mattered at all. As I said when I put a red dot on it I forsee it not cowitnessing
Ignore cowitnissing, both the irons and optic would have their own zero.
Any chance the magpul buis is canted when you installed it? Finding an experienced shooter is still ideal, BCM uses a bore laser to verify the FSB is straight but things do happen.
?I’ll try to get out again this weekend but I am pretty confident in both my and my brother in laws abilities. I have been an avid hunter for decades. I was shooting 1to 2 inch groups at 100 yards I just thought my irons would be confusing to look at through an aim point being so far off center. Perhaps this is a non issue as I am easily hitting what I am aiming at. I just expected a little more from BCM from all I’ve read on this site.
O.k. I stuck a red dot from one of my shotguns on it and while I can access the irons through the R.D. I do not like the fact that they are off center. I guess I imagined I could sight in the irons then put the dot on the FSP and all would be in order. Not so with this set up.
Shit happens. If you are getting 2MOA then your a good shot. More than likely the FSB is slightly canted, it wont make a difference with an optic. So you can either live with it(which wont damage the rifle) or contact BCM tomorrow and see what they say, make sure you tell them your fsb seems canted.
No matter who makes it, its still made by humans so things happen sometimes
What red dot on the shotgun did you use. Also while doing that will get you on paper usually the optic will have its own zero, BUIS are just that, back up iron sights.
Are you able to try a different rear sight? I’d be more inclined to believe the plastic Magpul sight is the culprit than the upper.
The only gun I have Magpul sights on is at work, or I’d take a look at mine.
Another tip is to make sure you have a consistent cheek weld on the gun, making sure to have a repeatable contact point such as the tip of your nose touching the charging handle. That way the distance from your eyes to the rear sight doesn’t change. This could effect zeroing.
Thanks for all your input guys, I have sent another e-mail to BCM letting them know the rear sight is 29 clicks left of mechanical zero to line up. I can see no issue with the Magpul by just looking at it. However as one of you mentioned, it is plastic. For that matter, I can see no issue with the FSB by just looking at it either. I will upgrade this as soon as something better can be ordered and recieved. I am determined to put together a quality weapon as I am in pretty deep already. The red dot I used is just a cheap bsa I use on a shotgun, I just wanted to put something on there to see what it would look like should the need to use irons arrise.
tag for answer. I have a similar problem: LMT upper (sighted fine on BUIS), but my trigicon 1.5x reticle is approx 15 degrees up and right of the FSB. Seems to be OK, but that blurry FSB being off-center so much really concerns me.
Exactly, makes it difficult to witness, for lack of a better term. I feel I would be better off without the FSB all together.
You should be able to get a carry handle cheap on Ebay or the EE then you can narrow down if it’s the mbus or the upper
I have Magpul BUS on my rifle as well and the rear is almost all the way to the left. I ended up having to have the upper receiver itself replaced and I had hoped that the sights would zero more toward the middle with the new one, but they didn’t change much at all. I finally decided not to worry about it. I got the rifle zeroed with the BUS and that was the primary concern.
On a side note, it seems every time I see this issue brought up about the Magpul BUS the rear sight needs to be moved far to the left. Never to the right.
This isn’t only related to MBUS, this seems to be a decently common question.
If you’re able to zero the gun, even if you’re near the edge of adjustment, you’re OK as long as everything in your upper has been put together correctly (barrel nut torqued right, sights installed tight and good lockup on the BCG/barrel extension).
I got it zeroed without any problems, and it still has some remaining adjustment to the left so I’m good to go. The upper as it was originally was factory assembled from DD. As it is now, it was assembled by a local guy who is top notch so I’m not worried about assembly. It bothers my OCD more than anything but I’ll just have to live with that.
Same with that TROY crap. There was a while were the same thing was popping up on their rear flipper too.
I have a BCM 20" A4 upper that with a DD A1.5, Matech, and Carry Handle needs to have the rear sight moved 13-15 clicks to the left.
This isn’t a problem with any other carbine/rifle I zero.
I have a feeling that BCM IS occasionally putting out uppers with SLIGHTLY canted FSB’s. I’ve read quite a few accounts of BCM uppers needing substantial left adjustment and it keeps being blown off as “a shooter problem” or a “rear sight problem”.
Well the guy is admitting to being new to the AR platform. And we want to work off facts as much as possible since these things have a way of taking on a life of their own.