View Full Version : Canted FSB or out of spec upper ?
So I took my BCM upper out today for the first time and I had to adjust the BUIS 33 clicks to center my shots at 25 yards. The rifle was shooting 3" to the right with the BUIS aperture centered. This BUIS was used on two different rifles and always centered my shots within a few clicks left or right so I know the BUIS is not the issue.
I took some measurements of the rail because I suspected that there might be an issue when my Larue mount from another rifle was loose when mounted to this upper.
Now I'm not sure if this needs to go back or not, that's what I'm here to ask. If it does I hope that BCM doesn't give me any issues because I had to cut off the bayonet lug for AWB compliance. I was able to take the lug off without the need to remove the FSB so I know that this in no way would mess up the alignment.
History: The upper was previously returned to BCM and replaced do to the upper being out of spec.
Images are attached and your opinions are really appreciated.
12101
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12103
Being the OCD perfectionist that I am, that would bother me. However if I push that aside and access the logical part of my brain, I would say that you are still within the functional limit of the sight. As long you can achieve a zero its good to go.
What kind of BUIS is that? You could try a different style BUIS. Something like the matech or KAC micro wouldn't be so noticeable if they end up so far from center.
Agree ^^^
The FSB base may be canted slightly to the left. You will find cases where the rear sight aperture is run all the way up against the left wall of the sight housing and still can't be zeroed. I have to believe that since "they" replaced the first one for being out of spec you shouldn't be surprised that this one appears to be no better.
Agree ^^^
The FSB base may be canted slightly to the left. You will find cases where the rear sight aperture is run all the way up against the left wall of the sight housing and still can't be zeroed. I have to believe that since "they" replaced the first one for being out of spec you shouldn't be surprised that this one appears to be no better.
Well the first upper was sort of crooked...
Original thread (https://www.m4carbine.net/showthread.php?t=104141)
"Sort of" appears to be an understatement. I would have to believe that since "they" machine all of their uppers and lowers in house, QC is getting a little sloppy. Maybe "he" will run in back and stop production until this gets sorted out. I'm sure a Mod or Staff type will be along shortly to remind you of Rule #4...
I doubt BCM is machining uppers and lowers in house. What is frustrating is having to send something back twice.
What's the problem? Its zeroed now isn't it....don't send it back, just get out there and shoot it. These sights have adjustability for a reason....this ocd must be exact dead Center craze seems to be popular at the minute. Re your other issue with your larue mount - again that can be adjusted to this rifle, so not really a problem.
DeltaSierra
05-06-12, 09:37
What's the problem? Its zeroed now isn't it....don't send it back, just get out there and shoot it. These sights have adjustability for a reason....this ocd must be exact dead Center craze seems to be popular at the minute. Re your other issue with your larue mount - again that can be adjusted to this rifle, so not really a problem.
*cough*
Good post...
Agree ^^^
The FSB base may be canted slightly to the left.
The other thing that it could be, which is common but often overlooked is that the front receiver face is very slightly out of true....which subsequently causes the barrel to be tiny fractions of a degree off center. Basically a bi-product of tolerance variation and high volume manufacturing. I have one rifle which is noticeably like this, and my rear sight is similarly adjusted as per the picture here. If I had the inclination to do it, I'd get a receiver lapping tool and true up the front face of the receiver to make sure everything was exactly perfect. But the fact is, so long as you can cover the adjustment with the sight, its really not that big of a deal and doesn't bother me.
This AR may have a problem though....
;-)
http://i.dailymail.co.uk/i/pix/2012/05/03/article-0-12E63B3F000005DC-151_306x423.jpg
If you have a carrying handle laying around put it on the upper. Then turn the whole thing upside down and see if there are four points of contact. Also make sure what you place it on is flat and true. If there isn't then there is a problem. On a brand new upper you should have all four points touching within a few sheets of paper. Also when you set the upper down flat how does the front sight base look? You paid money for it so it should be right.
Dan
Call BCM and tell them your problem and ask them to repair or replace. I'd bet you have an RMA# shortly and they swap out the upper. Who knows, you might have the first of a bad batch that made it out the door and needs to be stopped from being sold. Let them know ASAP so the problem can get resolved.
Had a similar experience with my BCM midlength, BUIS looks similar, the Aimpoint required considerable adjustment also. I think there are several threads concerning this issue with BCM middies. Looks like a QC issue to me.
When I took my Aimpoint off my other upper (the same upper that had the Troy BUIS) it took 24 clicks to the left and 24 click down to hit the point of aim.
SW-Shooter
05-06-12, 20:03
Wow, 2 in a row.
All this "crooked mid-length" stuff has me a little spooked. Mine just arrived last week and has yet to be fired. I flipped it upside down on the table and it's a little off, but not much. I boresighted it out the window (~25m) and it's close to mechanical zero, so I think it's fine. It may be a little while before I can get it out to the range and on the bench.. Ron
Wow, 2 in a row.
what was the other one?
Some of you guys must have some expensive kitchens to trust the flatness of your tabletop to determine if your upper assemblies are square.
OP, front sight posts are relatively easy to visually inspect if they're straight or not. I've had a couple FSB come crooked from a particular manufacturer (not BCM) and it was readily apparent by visual inspection.
If the FSB appears straight, and you have the experience/ability & tools to assemble uppers, you might consider removing the barrel nut & barrel from the receiver and simply re-installing. Same with your rear sight. You never know...
what was the other one?
SW-Shooter is on some big crusade to say that BCM is no better than DPMS, Olympic Arms, etc.
Seems to me perhaps BCM got a bad batch of upper receivers. This problem has been reported a lot lately. In all honesty, it's small potatoes in comparison to the problems companies like DPMS and RRA see buddy.
When I took my Aimpoint off my other upper (the same upper that had the Troy BUIS) it took 24 clicks to the left and 24 click down to hit the point of aim.
In that case, my money is on my point made in post #9 being the "issue", rather than a canted FSB. But I still hold the fact you can zero it means its not really a big deal. Even with two rifles that are completely true, you will have to re-zero an optic to some degree when moving it from one rifle to another.
If you want to try for perfection, remove the barrel and lap the upper receiver face using this sort of tool:
http://www.brownells.com/.aspx/pid=20220/Product/AR-15-M16-UPPER-RECEIVER-LAPPING-TOOL
Well BCM said that it was laser sighted to within 3 clicks using a mil-spec carry handle and for me to try a mil-spec carry handle to see what the results are. Since I don't have a mil-spec carry handle I can send it back to them on my dime ($20 or so) to have them check it and make adjustments.
My buddy has a Bushmaster carry handle but I don't know if that is mil-spec or not. I'm not really sure what to do at this point.
If you want to try for perfection, remove the barrel and lap the upper receiver face using this sort of tool:
http://www.brownells.com/.aspx/pid=20220/Product/AR-15-M16-UPPER-RECEIVER-LAPPING-TOOL
Of all the junkass AR-15 hobbyist tools that they sell, the lapping tool is king of the hill. The steel that piece of shit is made from is softer than the 7075-T6 upper. Coupled with the fact that you are abrading the tool at the same time you are ****ing up your upper, exactly what are you squaring to what?
Well BCM said that it was laser sighted to within 3 clicks using a mil-spec carry handle and for me to try a mil-spec carry handle to see what the results are. Since I don't have a mil-spec carry handle I can send it back to them on my dime ($20 or so) to have them check it and make adjustments.
My buddy has a Bushmaster carry handle but I don't know if that is mil-spec or not. I'm not really sure what to do at this point.
This info is correct (on them checking FSB's with a laser).
In the past, we have seen BUIS's with bad clamping brackets (don't engage the receiver properly and are canted to one side or another).
So using a known quality carry handle (like Colt) is a good way to check things out.
C4
Of all the junkass AR-15 hobbyist tools that they sell, the lapping tool is king of the hill. The steel that piece of shit is made from is softer than the 7075-T6 upper. Coupled with the fact that you are abrading the tool at the same time you are ****ing up your upper, exactly what are you squaring to what?
The front of the receiver face where it joins with the barrel. Its well known there are variations there. If you use the tool properly you are not ****ing up anything, you are literally shaving a fraction of a hairs worth of metal. Is it really worth doing? No in my opinion, but if you are going to quibble about having to adjust your sight off center, it'd be worth it to some.
I'm not really sure what to do at this point.
It looks like this is enough of a problem to you that the $20 for shipping is nothing compared to the ease of mind you will get in return. Just send it back, have them bless or fix it and be done with it.
It looks like this is enough of a problem to you that the $20 for shipping is nothing compared to the ease of mind you will get in return. Just send it back, have them bless or fix it and be done with it.
:D BCM just sent me a return label so they can inspect it. that's quality customer service right there.
SW-Shooter
05-07-12, 13:18
SW-Shooter is on some big crusade to say that BCM is no better than DPMS, Olympic Arms, etc.
Seems to me perhaps BCM got a bad batch of upper receivers. This problem has been reported a lot lately. In all honesty, it's small potatoes in comparison to the problems companies like DPMS and RRA see buddy.
You are taking one thread and molding it to fit your incorrect observation. If you do a simple scan the last ten pages in the AR forums General & Tech, you'll find that there have been no less than 3 instances (TOS as well) of out of spec BCM products (uppers). That doesn't mean I said "go out and buy DPMS", what that means is BCM is having so QC issues lately. With the rush it's bound to happen.
My point in the other non technical thread was that you should buy the best you can afford and become proficient with that. It's better to know your tool, than to sit there and admire it because you can't afford to shoot the darn thing. You took what I stated out of context.
I am not on a crusade against BCM, but it's the community's responsibility to make everyone aware of the problems they are experiencing. Every manufacturer has produced an out of spec part(s) at one time or another. That doesn't make the brand crap. That is my only point.
Ned Christiansen
05-07-12, 13:52
I don't see a rear sight off center to be an issue..... maybe, if it's bottomed out all the way to one side.
That's why we have adjustable sights on these things. If every AR had the adjustable rear sight perfectly centered, we could do away with the adjustability. But there are just too many connections and tolerances between the muzzle and the rear sight-- it would be impossible to get every sight perfectly centered:
Front sight to barrel mounting (or):
Front sight to rail mounting
Rail to receiver
Barrel to receiver
Receiver threads
Receiver face
Barrel extension
Barrel extension locating pin to slot in receiver face
Barrel nut
Rear sight to receiver mounting
..... not to mention, tolerances in the barrel itself with regards to concentricity and straightness.
Even with the very highest quality parts, and let's say, held to 75% of the tolerances, there are still just too many joints to hope for perfection every time. The one pictured is not what I'd call a problem but I think it's very kind of BCM to take a look at it.
I don't see a rear sight off center to be an issue..... maybe, if it's bottomed out all the way to one side.
That's why we have adjustable sights on these things. If every AR had the adjustable rear sight perfectly centered, we could do away with the adjustability. But there are just too many connections and tolerances between the muzzle and the rear sight-- it would be impossible to get every sight perfectly centered:
Front sight to barrel mounting (or):
Front sight to rail mounting
Rail to receiver
Barrel to receiver
Receiver threads
Receiver face
Barrel extension
Barrel extension locating pin to slot in receiver face
Barrel nut
Rear sight to receiver mounting
..... not to mention, tolerances in the barrel itself with regards to concentricity and straightness.
Even with the very highest quality parts, and let's say, held to 75% of the tolerances, there are still just too many joints to hope for perfection every time. The one pictured is not what I'd call a problem but I think it's very kind of BCM to take a look at it.
I agree that there needs to be adjustments but the spec according to BCM is 23 clicks either way so I just wanted to make sure there wasn't anything messed up
I gotta tell you, I've got a Colt 6520 that's the same way. As in 6520 with a fixed carry handle. If you can sight it in within the range of adjustment, it's good. FWIW, I've got a mid length 14.5" BCM with the same offset. I aint worried. These guns are going to be way more out of spec when I'm done beating the snot out of 'em.
Just to update:
BCM has sent the upper back and it should be here on Friday. BCM states that they "inspected and laser sighted" the upper. Not sure what that really means but I hope they just didn't bubba the sight into alignment.
polymorpheous
05-16-12, 19:28
Just to update:
BCM has sent the upper back and it should be here on Friday. BCM states that they "inspected and laser sighted" the upper. Not sure what that really means but I hope they just didn't bubba the sight into alignment.
BCM isn't known to "bubba" anything.
Just to update:
BCM has sent the upper back and it should be here on Friday. BCM states that they "inspected and laser sighted" the upper. Not sure what that really means but I hope they just didn't bubba the sight into alignment.
That means that they verified the the FSB has been checked with a laser to make sure that IT is not canted.
The words "bubba"" and BCM don't go together.
C4
That means that they verified the the FSB has been checked with a laser to make sure that IT is not canted.
The words "bubba"" and BCM don't go together.
C4
Thanks Grant.
When I say "bubba'd" I mean hitting the FSB into place with a mallet or shaving down one side of the receiver notch and peening the other. Their emails are so vague so I am left wondering what was done. Visually I could see that the FSB was canted so I guess I have to wait and see.
This thread has run it's course. Have you ever heard of BCM doing any such antics as you have described? I know I haven't. If you have further issues then follow the rules and let us know.
Thanks Grant.
When I say "bubba'd" I mean hitting the FSB into place with a mallet or shaving down one side of the receiver notch and peening the other. Their emails are so vague so I am left wondering what was done. Visually I could see that the FSB was canted so I guess I have to wait and see.
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