Could use some help on a new AR issue...


February 21, 2011, 12:44 AM
Hello everyone.

I just got done putting together a BCM uppered rifle. The upper came assembled with a BCG, flattop and a troy rail.

I mounted a set of troy flip sights on the rail and the rifle is shooting very far left. I Had to move the rear sight almost to its right side limit.

I have heard of this issue w/ canted FSBs but I dont have one, so...

An tips anyone can provide would help... I sent an email to troy thinking it is the sights, I hope I get a positive response.

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February 21, 2011, 03:55 AM
Is the rail aligned correctly?

February 21, 2011, 04:33 AM
Yes. The upper and the rail are square to each other.

Bartholomew Roberts
February 21, 2011, 07:56 AM
The same thing can sometimes happen when the barrel nut is torqued so that the index pin is pushing against the side of the upper receiver notch. I would do my best to eliminate other issues first (like sights) and if you still have the problem, contact BCM and discuss it with them.

February 21, 2011, 09:26 AM
Which Troy rail did you get? Just because the upper and rail look like they are square to each other doesn't mean that the rail in on there straight. It doesn't have to be off by much and the longer the rail is the farther off the sights will be with only a little crookedness (is that a word?).

When I attach a rail, I used a foot long piece of aluminum stock that I turned on my lathe to a diameter of 1" and I know is straight. I use two sets of scope rings with the piece of rod to make sure that the rail is going on straight and won't be cocked to one side at the end near the muzzle. I also know of guys using carry handles or even old scopes to accomplish the same thing.

It can actually be pretty tough to get a rail on perfectly straight.

February 21, 2011, 10:04 AM
If possible, mount a temporary scope to the reciever only, to eliminate a barrel issue. If the scope is mechanically zeroed, and the rifle shoots to POA, you can chase a rail or sight issue.

February 21, 2011, 12:14 PM
Well, first off BCM built the upper with the rail. So I'd hope the barrel would on right and same for the rail. But who ever said you get what you paid for hasnt tried it.

I will try the scope idea. I was going to do it with a bolt on rail w/ a2 sight my dad has, but that should work better.

Tony, I checked the rail with a level. Leveled the rifle on some sand bags by the UR flat top, then checked the rail level in three places. Looked good to me. I am sure the scope ring trick works better, but I dont have that kind of stuff laying around. Thanks for the idea though, This does seem like the most likely issue.

February 21, 2011, 12:18 PM
When you mounted the sights, did you hold them firmly forward and against the side of the base that is fixed in place (part of the sight frame)? Any chance one or both of the sights isn't completely square to its rail?

February 21, 2011, 01:19 PM
I held them foward and against the solid portion of the sight body.

The rifle shot approximately 6 inches left at 50 yards before adjustment. I worry that that is a lot of distance to be attributed to poorly installed sights.

Anyone have quality issues with BCM?

Also it is a troy 11' TRX standard rail

February 21, 2011, 01:27 PM
We've got the same uppers. I didn't have any issues with my Troy sights, other than it was hitting really low. Good luck.

Edited to add: BCM has an industry forum at; I would suggest you start a thread there and see what you can find. That, or email them tomorrow.

February 21, 2011, 01:34 PM
BCM is good stuff!

February 21, 2011, 01:40 PM
The rifle shot approximately 6 inches left at 50 yards before adjustment. I worry that that is a lot of distance to be attributed to poorly installed sights.

That actually doesn't sound like all that much to me. I would call BCM. I'll bet that the issue is some combination of the TRX rail being slightly off perfect alignment and the barrel itself being imperfectly square to the receiver top.

I have a flattop with a carry handle that is like that, with fixed A2 front sight and properly attached carry handle.

If the Troys are just BUIS, I probably would accept this alignment. But if it bugs you, and it seems to, call BCM.

February 21, 2011, 02:25 PM
im not sure if this is the problem, but you might have the rear sight tightened down on a cant.

I had the same problem with my Matech when i put it on my BCM, and was wondering why in the hell it was shooting a foot to the left!

(didnt figure out it was canted until i got home from the range a few hours later)
major "DOH!" moment.
re-adjusted and tighened the rear sight again and now its an MOA shooter to 300 yards with cheapie federal ammo.

I'd check alignment on the sights first, and then go into more in-depth work.

hope this helps,

February 22, 2011, 02:33 PM
The rear sight has a slight cant, that I cant get rid of. I belive it is too slight to make the difference I am seeing, unfortunately. I found the canting with a level, I leveled the flattop on some sandbags and compaired the rear sight by the ears and it was off maybe a half a bubble.

What is irking me is the rear sight is pulled so far to the right that it is almost to its limit. It actually makes sight down the rifle harder, I have to adjust my usual cheekweld.

Ive borrowed a carry handle i am going to give it a shot with, see if it is the rear sight...

February 22, 2011, 03:04 PM

I mounted the carry handle sight in between my 2 troys to see if they all line up when zeroed. Here is the result

February 22, 2011, 03:10 PM
I mounted the carry handle sight in between my 2 troys to see if they all line up when zeroed. Here is the result

How i mounted them-

Top view- you can sdee here if it was in your hand that they are not inline. Through pictures its tough though.


In the last picture you can see I dont have the sights perfectly aligned, but even so you can see the carry handle sight, at zero, is much more to the right then the troy at zero. There is also a slight elevation difference too, but I am ignoring that. As someone else said earlier, his set up shot low just as mine does. This is an easy fix that I am not worried about.

When I look down the sights and align them completely, its even worse. when I flip them to the small apertures I cant see the FSP at all.

I moved the carry handle sight to be mounted on just the rail as well and the issue appears to be the same, I dont belive it is canted (my level agrees)

Well, the next step is to get to a range to be sure.

February 22, 2011, 03:12 PM
If you look at the last pic, at the base of the troy compared to the carry handle behind it, it is leaning left. Okay... Now I am getting somewhere.

I tried to remount the rear sight over and over again. The cant wont go away. After the range trip, I belive I will call Troy...

Shawn Dodson
February 22, 2011, 05:40 PM
Mount your upper receiver, with bolt carrier removed, in a solid rest (or a receiver block).

Adjust your rear sight so it's centered.

Tape a piece of paper to a wall about 10-15 feet from the muzzle. Use a level to draw a plumb vertical line on the paper.

Look through the breech and align the vertical line so it passes from 12 o'clock to 6 o'clock in the bore. (You should also place the level on top of the rail on the receiver to make sure it's level in the rest or block.)

When you have the receiver leveled and the line passing from 12-6 o'clock as you look through the bore, then look through your rear sight and see how your sights align to the line on the paper. The line on the paper should pass straight into the top of the front sight post when you align your sights.

February 23, 2011, 05:51 PM
So I shot it today. The carry handle rear sight sighted in at 15 clicks right, same as my dads rifles w/ that sight.

The Troy sighted in with 32 clicks right. Its the canting (which is to the left)... it is slight, but must be the issue. I got to call troy I suppose.

February 23, 2011, 08:16 PM
this is very interesting....wondering what troy will say.i've only dealt with one brand of back up sights (magpul) had no trouble with them...but for what troy charges for there back up sights.....they should not have issues

February 24, 2011, 03:38 AM
Got that damned right. I emailed the Troy tech dept last friday... no response as of yet. Kind of irratated about that, Going to call tommorrow probably

February 28, 2011, 05:58 AM
Just as an update... After I didnt get a reply for a week from Troy, I called them and left a message. Got a call back the same day, and they sent me a label to sent em back on them. Thats pretty cool, Lets hope a new set works better.

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