My front sight tower is out of alignment. I run out of windage trying to zero the rifle at 50 yards. I tapped out the pins last night and straightened the sight tower. When I start to tap the pins back in the sight tower moves back to it's canted position. Anyone have any tips on how to fix this problem?

The most common remedy is to place your upper in a vise and use a rubber/leather faced mallet to give the sight tower a whack.

If your upper is under warranty, you can send it back to the manufacturer to have the barrel re-torqued.

This topic pops up from time to time. I read about it with great interest a few weeks ago since I was in the same boat (rear sight dang near touching the sight housing at a 50 yard zero). I finally got up the nerve to lay the barrel on a sand bag, hold the action in hand, and whack the crap out of the side of front sight tower with a rubber mallet. (You'll want to move the front sight toward the POI) A little too much so I whacked the other direction and ended up with a perfectly centered rear sight. Normally I wouldn't recommend anyone just whacking away on a gun with a mallet, but in my case it worked just like you hear about.

It’s not really the front sight you are trying to move; you are turning the entire barrel/sight assembly inside the upper (that is why your sight was still canted when you reinstalled the taper pins)

I had a similar problem...I had to take the front sight block off to get a RAS style float tube on...couple of days after I got done, I noticed the sight tower was not centered.  

I put the upper in an action block, and in the vise; then I took a big screwdriver, wrapped it in 3 or 4 shop rags, then stuck it into the sight tower, and just rotated the tower the way I needed to.  It took 2 tries, but I think it's perfectly centered now.  I thought about hitting it with a rubber mallet, but I ended up trying the screwdriver first...my reasoning was I less chance of bending or breaking it with the screwdriver.  

Is this really the best remedy or option?  Is there anything else that can be done, other than beating on the front sight tower & barrel with a rubber mallet, or prying with a screwdriver???  I'm in the same boat now, but somewhat reluctant to pound away.  Thanks.

Well, I had thought the thing would line up correctly when I drove the pins in, but it didn't.  At that point, you're gonna have to move it somehow, which involves imparting some sort of torque to the site tower to get it to spin on the barrel...if you can think of a different way, go for it.  

I assure you the screwdriver method I used didn't do any damage to the sight tower, and wasn't very traumaitc to me either lol.  

[i]Is this really the best remedy or option? Is there anything else that can be done, other than beating on the front sight tower & barrel with a rubber mallet, or prying with a screwdriver??? I'm in the same boat now, but somewhat reluctant to pound away. Thanks.[/i]

Sure.  If the lateral movement is not enough to cause alignment problems (how much is too much? - I don't know) with the gas port you can throw away the 2 taper pins that hold the tower in place, then drill/tap the bottom of the sight tower for set screws. This is the method many gas blocks use for attachment to the barrel

[b]Another idea[/b]
DISCLAIMER - I've never done this but it makes perfect sense:

I had thought about pulling the barrel off the upper and noting the amount of side to side movement available where the index pin engages the upper and shim/adjust accordingly to ensure that the front sight tower is centered when the barrel is re-installed. But the big mallet is much quicker.

More likely the front sight base is on the barrel straight, the barrel is twisted in the receiver.  This happens often when a barrel is installed using barrel vise jaws.  

Is the front sight base canted to the shooter's right?  The rear sight blade way over on the right side?

The receiver was twisted on the stationary barrel when the barrel nut was tightened.  The hard steel indexing pin dents the soft aluminum of the slot in the receiver.  

This can be remedied by removing the gas tube, placing the upper in an ACTION BLOCK.  Loosen the barrel nut and slide if forward.  The barrel is now loose.  Cut a sliver of shim stock and tap it (use a punch to do this) between the indexing pin and the receiver slot on the right side.

Tighten the barrel nut, replace the gas tube.  I'll bet it is all better now.

Hmmm mallet or screw driver, mallet or screw driver. I have an vise block on the way, so maybe I'll wait and tear it down then, or maybe I'll loose patience and wack the crap out of it.

This can be remedied by removing the gas tube, placing the upper in an ACTION BLOCK. Loosen the barrel nut and slide it forward. The barrel is now loose. Cut a sliver of shim stock and tap it (use a punch to do this) between the indexing pin and the receiver slot on the right side.
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The indexing pin is pretty tight in the receiver slot, and there doesn't seem to be any room for any shim stock.  Any other suggestions?  Thanks.

I have some theoretical questions...how is the sight tower ever going to remain stationary/aligned where you want it after you beat the pins back in to secure it?  And how is the barrel itself spinning when all the pressure/torque goes on the barrel nut, assuming you're using and action block?  I don't see where the force to turn the barrel enough for the indexing pin to dig into the receiver groove and spin the whole shebang is gonna come from.  

And, it seems to me that if the barrel is gonna move, it'll move in the direction the torque is applied...so if anything, the sight should be canted to the LEFT, not right.  

It took 7 or 8 good whacks with a regular hammer to get each of those pins in...driving them in from the right side would rotate the front sight to the right, since the pressure from the hammering is coming on the bottom of the sight tower, which should spin the top of the sight as the bottom moves...IF it moves.  

My question, then, is what are the chances that the tower will actually be straight after you get the pins in completely?  Wouldn't if be fairly common to have to "tweak" it a hair?  If it's the barrel that moves, then wouldn't the barrel be "out of time", like whan an FAL barrel is not torqued to 12:00?  

I'm asking these questions as I'm "thinking out loud", not to prove anyone right or wrong.  Just interested in what's going on as the barrel gets torqued down.  

IF the upper was assembled with barrel vise jaws, then it is possible for the RECEIVER to be twisted, as the barrel nut is tightened, so that the receiver is to the shooter's left, the barrel being held stationary.  Therefore the FSB is canted to the right.

To have the receiver clamped in an action block, and then hammer on the FSB, would be most foolish.  The receiver is aluminum, the barrel is stiffer steel, and at that distance, the leverage is way too much to not bend the receiver.  DON'T DO THIS.

When tapping the pins in, the barrel should be flat on the workbench, with a machinist's block under the FSB to support it.

I ask again, which way is the FSB canted?

There's a much better way than taking a big hammer and beating on your front sight.

This is actually a very common problem with homebuilts.  I can't tell you how many FrankenARs I've seen at the range with the rear sight cranked over 20 or 30 clicks in order to sight in an obviously canted front sight base.

And here's how you fix it -

Unscrew the barrel nut and line up the front and rear sights. I do it by eyeball. You can also tie a piece of thread through the rear aperture to the front sight post.

If slants to one side, take a jeweler's file and with very light strokes, open up the barrel pin notch in the upper receiver. When you get the front sight where you want it, take a hammer & punch and very lightly peen down the loose side of the notch with the barrel in place. There should be no wobble. Remember - light taps!!! It doesn't take much.  (Otherwise the barrel may turn while you're tightening the barrel nut and you'll be back where you started.)

So basically what you're doing is moving the notch in upper receiver over slightly in order to bring the front sight straight up.  Easy, huh?  And you don't have to whack the crap out of anything to do it.  [:D]

I used an action block to barrel my receiver, not the barrel jaws.  

My FSB was canted to the right, from the shooter's perspective, about 2 degrees.  I'm not trying to be argumentative, but no one has articulated it to me so that my feeble brain can understand lol...how can the barrel spin to the right, taking the FSB with it, when the barrel nut gets tightend with the receiver in an action block in the opposite direction?  If you look down the muzzle, visualize tightening the barrel nut...it spins right/clockwise...which should spin the barrel in the same direction, IF it moves.  That would also take the FSB right as you look towards the receiver...so the sight, if canted, would be to the shooter's LEFT.  

First off, there shouldn't be any torque applied to the barrel, if I've got the wrench on the barrel nut, and the nut screws over the threads on the receiver.  

I could see the FSB/barrel spinning left, in the direction that the barrel nuts gets tightened, but not right.

What am I missing here?    

I don't think you missed a thing about the action block or the barrel vise blocks, it just might be that the front sight was installed a hair off. Since you used the action block and the front sight is still to the right (as seen from the rear sight?) maybe you need to follow the procedure posted by Homo_Erectus and do a little filing on the left side of the index pin notch till the front sight is [s]right[/s] I mean correct. I've done this and it worked for me only instead of peening down the loose side of the notch (which I didn't think of) I was lucky and used a sliver of shim material to hold the index pin in the right position.

Homo_erectus, not just homebuilt Frankenguns, factory AR's, too!

Fenian, dunno, it happens.  I agree with ARFan, do what Homo_Er says.

OK, mleaky wrote me his FSB is canted to the right.  I wrote back as follow:

Ahah!!!  What happened, chances are it was assembled with vice jaws holding the barrel.  As the barrel nut was tightened, it twisted the receiver hard to the shooter's left.

IF you remove the barrel nut, you will find that the index pin is pushed into the soft aluminum.  You can do as I said, with some thin shim stock, and it will be just fine.  

(Or do what Homo_Erectus suggests)

OK, now, ask me how I am such an expert on this... I have EXPERIENCE. And you know what experience is?  It means, I f---ed up like that already!  Been there, done that.  

OK, you can also fix it by holding the receiver in an action block.  Make damned sure you use the insert that comes with it, that plugs in where the bolt carrier goes.  Chuck it up in the vise, and with the padded screwdriver in stuck in the front sight base, twist in the correct direction.  You should feel the pin hit the other side of the slot.

But if it can twist, it can twist back.  It should be OK, though.

Oh, you STILL need barrel vise jaws... to hold the barrel while you are installing or removing flash hiders. That way you don't twist the barrel in the receiver, which will also happen if you do that using an action block.

Action block for installing barrels.
Vise jaws for installing flash hiders.

Now, if you decide to remove the gas tube and undo the nut, before you remove the barrel nut, mark it (a tiny scratch is ok, a dot of paint, whatever) so you know which notch the gas tube passed thru.  

This is the no torque wrench method, and is perfectly OK.  

With the receiver in the action block, back off the barrel nut.  

You can feel the index pin hit each side as you twist the front sight base each way.  

Oh, yes, support the end of the barrel, don't let it hang out there without the nut on it.

Get some thin shim stock, and cut a little sliver.  Slip it between the index pin and the right side of the slot.  You will need to use a punch or an ordinary carpenter's nail set, and a small hammer, or even just the handle of a screwdriver, and tap the little piece of shim in there.  You can do this tapping lightly so as not to damage anything.  

(If you can't get thin shim stock in there, peen it over as per Homo_Er)

Now, with a little wipe of ordinary wheel bearing grease on the threads, tighten the barrel nut by hand.  Look at the front sight base by sighting from the muzzle end.  Is it straight?

OK, now using a breaker bar, not a torque wrench, snug the barrel nut back to where it was before, you marked the notch to know where.  Smooth and easy does it.  Now replace the gas tube and pin.

Done.

OK, after reading these suggestions, I'm ready myself to try fixing my own canted (to the right) front sight tower.  Thanks.

Also, anyone have an extra preban (with bayonet lug) Colt gray front sight tower they'd like to sell?

And does anyone have any tools for working on AR15's for sale?

Thanks.

The sight tower and barrel are drilled together in a jig for the 2 taper pins.  Your odds of finding an already drilled tower that will match up to your already drilled barrel are very slim.
The simplest fix for a canted/tilted/off-center tower is to drill and tap 2 set screws on the bottom and forget the taper pins.

I had the same problem and shiming the receiver
is the best way to go thats what i did and it worked perfect beating the front sight is not really the way to go

I got 1 off eBay.Came off an M4. absolutly no marks on it.He provided taper pins as well.