What causes canted FSB?

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[ARCHIVED THREAD] - What causes canted FSB?

Posted: 10/27/2003 9:14:29 AM EDT

Just got an upper and went to sight it in today and it shoots WAY left...looks like the fsb is canted. I got the upper used and it's had a float tube put on, it's a colt hbar 20. Just wondered what makes the fsb cant like this.

Posted: 10/27/2003 9:53:04 AM EDT

[#1]

what is the B in FSB?

if the barrell was removed to install the float tube, sometimes they don't realign the barrel, perfectly. I have one that is a little canted but nothing that I cant adjust with windage adjustments.

Posted: 10/27/2003 5:26:09 PM EDT

[#2]

Canted front sights are almost always a misalignment between the barrel assembly (of which the FSB is a part) and the upper receiver. There is a notch in the receiver, and a pin on the barrel that fits in the notch, that is supposed to keep the barrel from rotating in the receiver as the barrel nut is tightened. In the real world, if you aren't careful, you can transfer too much rotational torque to the barrel and deform the pin, allowing the barrel to rotate a degree or two. That is a LOT of misalignment when it is exaggerated by the 2" distance from the center of the bore.

There are techniques that can be used to minimize barrel rotation (applying moly lube in the right locations before tightening the barrel nut), and there is a "guage" (really a rod with a conical point on the end) that helps you verify that the barrel is properly aligned with the upper.

-Troy

Posted: 10/27/2003 7:43:05 PM EDT

[#3]

Quoted:
what is the B in FSB?

View Quote

FSB = front sight base

Another cause is that it came from the factory that way.

Posted: 10/27/2003 9:19:28 PM EDT

[#4]

hmmmm.... I've always know it to be the Front sight tower. I knew what he was talking about though.

Posted: 10/27/2003 11:52:07 PM EDT

[#5]

Quoted:
what is the B in FSB?

View Quote

[url=http://ar15.com/forums/announcement.html?b=3&f=66&id=188]Glossary[/url]

Posted: 10/28/2003 6:32:38 AM EDT

[#6]

Yeah, the weird thing about this is that it's canted to the right. I would think if it were overtorqued on installation and the alignment pin bent it would be canted to the left? Maybe when the FSB pins were reinstalled they were hammered a little too hard, enough to roll the sight base slightly to the right?

Posted: 10/28/2003 8:24:39 AM EDT

[#7]

Quoted:
Yeah, the weird thing about this is that it's canted to the right. I would think if it were overtorqued on installation and the alignment pin bent it would be canted to the left? Maybe when the FSB pins were reinstalled they were hammered a little too hard, enough to roll the sight base slightly to the right?

View Quote

If an action block/upper vise block is used to mate the barrel to the upper receiver the FSB will cant to the right. This is what most people use for mating these two parts.

If a barrel vise block is used, it will cant to the left.

Posted: 10/28/2003 8:37:54 PM EDT

[#8]

gee I thought canted FSB where just limited to the SAR AKs....this is the first I heard of a AR with a canted FSB.

what did colt do, import a bunch of gypies from romania to assemble their rifles?

Posted: 10/29/2003 1:15:25 AM EDT

[#9]

Quoted:
what did colt do, import a bunch of gypies from romania to assemble their rifles?

View Quote

You MUST be an AKer. [:D] Colt doesn't have this problem.

Posted: 10/29/2003 5:55:53 AM EDT

[#10]

[romanian accent]Ve hab vays of making your rifle shoot sidevays[/romanian accent]
[smash] <----- Romanian gunsmith

Actually I like my AK's too, my romanian SAR1 doesn't have a canted FSB...lol. Oh well, I doubt it came from colt this way, but it's a nice rifle anyway, I'll get the sight fixed up, just needed opinions on what to do to fix it...so is there any way to tell if the pin is bent when I take it out of the receiver? Also...if using a receiver block commonly causes this problem how is this to be avoided?

Posted: 10/29/2003 10:42:28 AM EDT

[#11]

Using receiver blocks (instead of barrel blocks) is the RIGHT way to install a barrel, BUT regardless of how you install it, there are a few things that you need to do to ensure that the barrel is properly aligned.

First, you need to have some moly-based lubricant for the barrel nut, and you'll want to apply it both to the threads and to the collar at the bottom that rubs against the collar on the barrel. This helps reduce the torque applied to the barrel.

Second, make sure that the pin is in good shape, and replace it if not. You can also use small squares of aluminum cut from soda cans to shim the pin.

Third, if possible, have someone support the barrel while you tighten the barrel nut. You should tighten and loosen the nut twice before doing a final torque on the third time. This helps seat the threads properly and reduces the chances of the barrel being loose.

Finally, you can use a guage (if you have a flattop, you'll need a detachable carry handle). Brownells sells them, but it's really a piece of drill rod with a pointed end that you clamp into the channel on the carry handle. If the point is pointing straight at the front sight, you're good. If not, then you know how far the alignment is off, and can shim or adjust the barrel as needed.

-Troy

Posted: 10/29/2003 1:44:04 PM EDT

[#12]

Thanks for all the good info guys...I should be able to get it straightened out now. Appreciate it.

Jason

Posted: 10/29/2003 9:43:26 PM EDT

[#13]

Quoted:
Finally, you can use a guage (if you have a flattop, you'll need a detachable carry handle). Brownells sells them, but it's really a piece of drill rod with a pointed end that you clamp into the channel on the carry handle. If the point is pointing straight at the front sight, you're good. If not, then you know how far the alignment is off, and can shim or adjust the barrel as needed.

-Troy

View Quote

Ooooooo! That sounds neat, got a link? I have no idea what to search for on their website.

Posted: 10/30/2003 4:24:56 PM EDT

[#14]

I just looked and didn't find it (maybe discontinued?), but it was WAY too expensive anyway. Find a local machine shop and have them machine a point on a piece of stainless rod that will in your carry handle.

-Troy

Posted: 10/30/2003 8:21:22 PM EDT

[#15]

Quoted:
I just looked and didn't find it (maybe discontinued?), but it was WAY too expensive anyway. Find a local machine shop and have them machine a point on a piece of stainless rod that will in your carry handle.

-Troy

View Quote

Oh well, I only wanted it if it wasn't going to put too much of a dent in my wallet anyway. Thank you for taking the time to look.

Posted: 11/19/2003 9:07:49 AM EDT

[#16]

Hey kicker9898, did you get the canted sight problem fixed, and how hard was it to do it yourself?

Posted: 11/19/2003 9:20:20 AM EDT

[#17]

Oh yeah...wasn't hard at all. Just followed Troy's advice, put new Moly lube on the threads, had someone hold the barrel so it wouldn't have a tendency to "follow" the torque...lol. Not a problem, shoots straight as can be. But, I also ended up replacing the HBAR with an M4 Dissy barrel, like it much better now. Now the only problem is that I have a spare barrel laying around begging for a project upper...lol.

Jason

Posted: 11/19/2003 9:41:09 AM EDT

[#18]

thanks for the update. Looks like I'm going to have the same problem as you, I'm going to replace my 20"HBAR with an A2 profile barrel. the HBAR is just too heavy for me.

Posted: 11/19/2003 12:58:58 PM EDT

[#19]

Two more questions, how many pounds of torque do I use on the barrel nut when I put the new barrel on, and after the barrel is installed do I check the headspace?

Thanks 556Cliff

Posted: 11/19/2003 1:46:38 PM EDT

[#20]

On the third tightening, tighten to 35 ft/lbs., then continue tightening until the next notch/hole lines up with the gas tube hole in the upper. You should not exceed 80 ft/lbs.

-Troy

Posted: 11/19/2003 3:18:30 PM EDT

[#21]

Thanks Troy,

That answers that question but after the barrel is installed do I need to check headspace?

Posted: 11/19/2003 5:26:28 PM EDT

[#22]

In an AR, headspace occurs between the barrel assembly and the bolt. All you need is these two components to check the headspace of the barrel. Installing the barrel into an upper won't change this in any way.

-Troy

Posted: 11/19/2003 7:32:21 PM EDT

[#23]

Quoted:
On the third tightening, tighten to 35 ft/lbs.
-Troy

View Quote

It's 30ft/lbs according to the TM. Not trying to be a "corrections jerk", just that for MANY people 30-35ft/lbs is where it lines up and not again until 90-100ft/lbs.

Posted: 11/19/2003 8:11:47 PM EDT

[#24]

Quoted:
In an AR, headspace occurs between the barrel assembly and the bolt. All you need is these two components to check the headspace of the barrel. Installing the barrel into an upper won't change this in any way.

-Troy

View Quote

So is that a NO I don't have to check headspace[?]

Posted: 11/19/2003 10:37:36 PM EDT

[#25]

Checking the HS is not necessary (but not a bad idea) if the barrel and bolt have been HSed previously. Switching the barrel and bolt into a new upper will not change the HS. Changing the barrel and/or bolt can.

ETA, wyv3rn's got the torque value correct according to the latest 23&P C6. The different values come from different manuals or different changes to the manuals. Doesn't matter much, tight is tight, loose is loose. 30 ft/lbs isn't a lot of torque.