FSB cant?

AR/M4 AR Technical Discussion
1

I know there has been a lot of “canted” FSB threads the past few months. I never knew I was going to make one until today. While out shooting my newly built 20" BCM, I noticed something. Whether it was an illusion or not was hard to decide. While looking through the rear sight, and while looking through the bottom of the DD sight, it appeared to me that the rear sight was canted to the left. This could explain why I was only a few clicks from being all the way to the left on the rear sight, and still being unable to get a good zero at 100. I have perfect windage under 25 yards, but as soon as I go to 100, the group stays right.

Tonight I took the rear sight off and looked straight down the upper. It still sort of appeared left, but other times it didn’t. I tried the “balance the upper upside down on the sights and slide paper under the FSB” test. Well, low and behold, with the DD sight on, I was able to slide paper under the right wing of the FSB. (I should note that when I put a carry-handle on, it did not come off the table as much, if that says something about the DD sight).

With the MOE handguard on, the top holes are slightly canted to the left of the FSB, not perfectly in line. (No pics of this) I looked for 20 minutes in every angle imaginable and I still can’t decide whether its canted. Sometimes it does look canted left, others it doesn’t. At some angles it looks canted right even. I’m usually not very OCD about things, but this really is bothering me. I don’t want to send the upper back to BCM just to be told its aligned, but then again I don’t won’t a misaligned FSB if I am right.

I know its nearly “impossible” to tell with pictures, but I guess it worth a shot. What do you think?

Looking at the open hole on the bottom of the DD sight, looks like a right cant:

Camera sitting on upper receiver:

To me it appears that the handguard cap isn’t aligned with the line going down the FSB. Illusion?

Off balance:

2

Look through it with both eyes…meaning shouldering the gun from both shoulders. Sometimes I think optics are canted when they are not because I have a natural cant.

Did you contact BCM?

Don’t use plastic handguards to check for cant.

3

Not yet, I wanted a few opinions on here first, then contact them.

I wasn’t really basing it on the handgaurds, more of a supplement to what I have already though.

I will try what you said.

4

It’s really difficult to tell by just the pictures, but it looks straight to me.
I copied your second and third picture into MS Paint and drew straight lines down the gas tube to the front sight post…very un-scientific, but it still looks straight…at least to me.

5

I just went back and looked again. It appears to me that it’s just an optical illusion to me that it’s canted. If I look through my right eye on the bottom of the sight, it looks canted left. If I look through my left it looks canted right.

I tried the paper trick again. I wanted to find out why paper couldn’t go under it with the carry handle but not the DD sight. It appeared to be because the sight aperture wasn’t flush with the sight wings, so it and one of the winds sat on the table, not both wings, causing it to be unlevel. This time I couldn’t get paper under it.

I guess I was worrying over nothing. Thanks guys.

6

In the future let’s contact the manufacturer first. I think some people are getting spun up about nothing. If there is an issue BCM will fix it as they always have, even when it wasn’t their issue.