canted front sight question

TWR · #1 08-12-06, 23:33

First let me say it's nice to have a place to go to for info without so much BS.

I have a Colt 16" lightweight barrel that I put on a flat top upper (CMT), when sighted in the rear sight is to the left of center. My first question is which way should I move the FSB/barrel to center it?

My second one, I've read that you can use a leather mallet to move the FSB/barrel in the reciever but that just doesn't make sense because of the index pin and barrel nut. Is this BS or is it an acceptable way of doing it?

Thanks in advance.

K.L. Davis · #2 08-13-06, 00:58

Bullet point of impact moves the same direction as you move the rear sight, and opposite of the front sight.

There is sometimes a little wiggle room for turning the barrel, some index pins have a little room to move in the receiver slot -- but hopefully you dont have much room to play with.

Sad thing is that some FSB are on a little crooked... that said, if you have a removable rear sight, that can be the culprit too -- in that the rear sight assembly may not be true center over the receiver.

Take a look at the rear sight, make sure that at mechanical zero it is over center of the receiver... One manufacturer had a real problem wuth this (well, more than one).

How far off is it when the rifle is zeroed? It is not uncommon to be off a little.

The best solution for centering the front sight is an adjustable FSB... but that is seen mostly on guns that are built with accuracy as the number one concern.

#3 08-13-06, 02:08

I've had this happen to me a few times over the years. I used a rubber mallet twice to correct the problem with no ill effects. You do have to give it a real hard whack for it to move. Just be sure and hit the tower square on the side. It will only move a couple thousands but sometimes that is enough.

The proper method would be to remove the barrel and fix the tower. Some guys do not like to send their rifles out me being one of them. Your best bet would be to send it to a smith and have them correct the problem.

Cheers

svtar15 · #4 08-16-06, 14:22

We had that on a couple barrels a while back. The reason this happens is the lack of meat that the original drill has to go through. On an HBAR, there is more material to drill through, which means there is more for the pins to hold on to. On a .625 diameter, sometimes the pin gets forced through without ever seating in the right spot. That's when a good whack with a rubber mallet will take care of the problem. Otherwise, all of the points I read above are legitimate causes also. You just have to play around with it and see what happens.

TWR · #5 08-16-06, 19:35

Thanks for the replies. It's not off that much, probably a little more than 3/4 to the left so it's not that big of a deal but it bugs me. It also was the same way on the A2 upper before I switched it so you're probably right about it being the fsb and not the upper.

I may try the hammer method but it still sounds like a "pull my finger" kind of thing. I've also been thinking of a PRI flip up, that would kill 2 birds but I like a pinned FSB. Anyway thanks again.

kart racer · #6 08-16-06, 21:50

I bought a stag upper and lower last month and had to adjust the buis almost all the way to the right.I sent it back,it's a little better but still not where I would like it.It's an arms 40 buis if it makes any difference.
I also have a carry handle that I haven't tried yet.I'm gonna try it, see if any of it was in the rear sight.

docmcb · #7 07-20-07, 02:49

This is where my rear sight ends up after using a laser boresighter. It's a new build and I haven't been to the range yet. Of course, result on the target paper trump the laser boresight, but if this is where my windage setting ends up, should I look into disassembly and reassembly of the upper to get the pin on the barrel more exactly centered in the receiver slot and correct barrel nut torque, or should I consider a little persuasion with a rubber mallet, or is this a common occurence, everyone's is a little off and I shouldn't be obsessive about it?

Apologies if you saw me asking the same thing at arfcom.

Robb Jensen · #8 07-20-07, 06:22

If you had the barrel clamped in vise blocks (and not the upper rec.) and you then torqued the barrel nut down you can get some weird windage problems like that. I use the DPMS 'claw' upper receiver block for installing/removing barrels.

TWR · #9 07-20-07, 09:09

It seems most cant issues tend to be toward the left. Kind of makes me think overtorquing the barrel nut and twisting the barrel, might be a big problem with homebuilds.

Snake RAH · #10 07-20-07, 11:25

Quote:

Originally Posted by TWR

It seems most cant issues tend to be toward the left. Kind of makes me think overtorquing the barrel nut and twisting the barrel, might be a big problem with homebuilds.

BINGO.

AR15barrels · #11 07-21-07, 03:26

Quote:

Originally Posted by TWR

It seems most cant issues tend to be toward the left. Kind of makes me think overtorquing the barrel nut and twisting the barrel, might be a big problem with homebuilds.

Read through my posts in this thread

TWR · #12 07-21-07, 08:13

Please don't ever link this site to the folks over there.

PALADIN-hgwt · #13 07-21-07, 09:51

xxxxx

kel3at · #14 07-21-07, 22:27

I built an AR using a Sabre 16" barrel with a CMT flat top M4 upper. I had the same problem, I had to correct for windage almost all the way left on my BUIS. I thought the FSB was canted, but it turned out the upper reciever was faulty. I took the thing apart many times trying to correct the problem before I realized what was really wrong. I sanded a little off the tab in the upper so I could spin the barrel a little, that corrected the sight but my M4 feedramps didn't line up properly and I could see that my FSB was now canted. I finally gave up after a year of tinkering and bought a whole new upper reciever. Guess what the new one lined up perfectly and I didn't have to put more than a few clicks of adjustment on my BUIS.

I sent my old upper back to CMT with a letter explaining what was wrong with the thing. I didn't call them, I just sent the thing back. A couple weeks later I got a brand new reciever in the mail. And now that reciever turned into a new gun

So it may be a long shot but make sure your upper is not faulty before you start pounding on your FSB. If your FSB is canted you should be able to tell by eyeballing it with your Foward handgaurd installed. If it looks straight up while you are looking down your handgaurd you could have a not so flat, flat top.

carbinero · #15 04-15-09, 16:29

I bought an upper which takes several more than 21 rear clicks to the left. I purchased it without BCG or BUIS, but the vendor wants those sent...(option "a")...is that logical, or should the canting be able to be remedied without needing those?

So I'm thinking about (b) calling around town for a knowledgable gunsmith (probably cost more than shipping, but would keep it local), or...

(c) smacking it myself with a rubber mallet, since that's what Grant says most guys do anyhow. If you'd advise me to try that, what do you clamp in the vice, the upper receiver?

ETA: I found the other thread on this: http://www.m4carbine.net/archive/index.php/t-18088.html ...So I think I'll try "C."

pertinent quotes: C4IGrant08-19-2008, 08:43 AM (in reply to: That wouldn't damage the upper receiver?)

"No. Just as an FYI, this is how all manufacturers get FSB's straight.

"

C4

"I have the upper secured and then simply hit the FSB to the left or right (depending on which way it needs to be moved)."

Puffy93 · #16 04-15-09, 16:37

Quote:

Originally Posted by TWR

It seems most cant issues tend to be toward the left. Kind of makes me think overtorquing the barrel nut and twisting the barrel, might be a big problem with homebuilds.

I thought that instaling the barrel nut was the first thing to a barrel(after rifling and chamber are cut of course) and then the gas port was drilled and FSB was installed.

jtb0311 · #17 04-16-09, 11:49

Quote:

Originally Posted by Puffy93

I thought that instaling the barrel nut was the first thing to a barrel(after rifling and chamber are cut of course) and then the gas port was drilled and FSB was installed.

Many home builders use barrels that already have the gas port drilled and FSB installed.

AR15barrels · #18 04-27-09, 22:51

Quote:

Originally Posted by Puffy93

I thought that instaling the barrel nut was the first thing to a barrel(after rifling and chamber are cut of course) and then the gas port was drilled and FSB was installed.

The barrel EXTENSION determines where TDC (top dead center for non car guys) is located on a barrel.
Gas port, markings and FSB alignment all key off the barrel extension.
The barrel nut just holds the barrel in the receiver.