I've seen topics on this, but of course can never remember what to do.
I finnally took my first personally built AR out to Zero, and it is shooting awesome, but the rear apperature is almost all the way left.  It is a flatop 16" M4gery with detachable carry handle.  Did I do something wrong with the front sight? barrel? rear Sight?

thanks

Assuming you have a standard FSB, and not a gas-block:

This happens when the slot in the upper receiver is notched a little off center, or when the FSB is not fully aligned with the nub on the barrel extention.  Hence, your front sight is "canted" to the left... making you have to adjust your rear sight all the way to the left.

The fix is... to remove the barrel, and slightly remove metal from the upper receiver (using a small round file), where the barrel extension pin sticks into the receiver.  Then, using a center punch, stake the left side of the receiver slightly, to "push" the pin on the barrel to the right.  Then, re-assemble the barrel nut and upper.  

You should be able to "eyeball" the canted FSB, with the carry handle or flat-top.... or using a laser bore sighter comes in handy for this process.  in any case, the goal is to *rotate* the barrel to the right, in the upper receiver.  Make sense?

It sounds a little difficult, but its really simple.  Just go slow.  You can get the rifle where it is dead on and the rear aperture centerd, with a little practice.

All back together and she runs great, shoots awesome

Thanks

Great!  I had one of these the other night.... rear sight had to move all the way to the left.  We filed the right side of the receiver notch, staked the left side, then tightened her down, and she was dead on, right in the middle.

I have a question.  Why is it that I have NEVER heard of people receiving complete rifles or complete uppers with this problem?  There must be a more precise way to make sure that the front and rear sight are veritcally aligned.  I'm sure that the previously mentioned fix works "fine", however is still seems like to could leave room for error.  I don't know about you guys but I would like my rifle to be pretty much ZEROED after a build.  Maybe this is a rediculaus thing to say, I don't know.  But to those who would scoff at this statement I have only question; "How do companys work it so that their rifles are zeroed without having to adjust the rear sight?".  In my opinion if your frontsight is canted, something was wrong with your build. (something out of spec, not put together right.... something!)

Quoted: I have a question.  Why is it that I have NEVER heard of people receiving complete rifles or complete uppers with this problem?

You havent been watching.  Several people over the past few years have reported this with factory rifles.... many have sent them back to Bushmaster to be repaired.

There must be a more precise way to make sure that the front and rear sight are veritcally aligned.  I'm sure that the previously mentioned fix works "fine", however is still seems like to could leave room for error.  I don't know about you guys but I would like my rifle to be pretty much ZEROED after a build.

Me too... but good luck with that!  Thats they there is this windage knob.

Maybe this is a rediculaus thing to say, I don't know.  But to those who would scoff at this statement I have only question; "How do companys work it so that their rifles are zeroed without having to adjust the rear sight?".

I dont think they do.  Personally, I have never seen a factory rifle, that was zero'd with NO adjustment on the windage knob.  If that happened, you just go lucky.  Every one of my Bushmaster uppers from the factory took SOME adjustment on the Windage knob during sight in.

In my opinion if your frontsight is canted, something was wrong with your build. (something out of spec, not put together right.... something!)

Possibly..... either the index pin is not perfectly aligned with the FSB, or the receiver slot was not cut perfectly.  And this is the fix.

At the factory, if they test for this, they can keep swapping receivers and barrels until they are happy.  I dont have this luxury.....

FALARAK,

What do you mean when you say "using a center punch, stake the left side of the receiver slightly, to "push" the pin on the barrel to the right"?

The rest of the description makes sense, but this part was not clear. I have a 20" upper that has the FSB canted to the left and if this is a fix to the problem, than I would be pleased to get it taken care of.

Quoted: FALARAK, What do you mean when you say "using a center punch, stake the left side of the receiver slightly, to "push" the pin on the barrel to the right"? The rest of the description makes sense, but this part was not clear. I have a 20" upper that has the FSB canted to the left and if this is a fix to the problem, than I would be pleased to get it taken care of.

Basically, you need to open up the right side of the receiver slot a bit... with a round file.  Once you do that, there will be a little more "slop" now, that the barrel can rotate in the receiver more to the right, and then back to the left.

We want to limit this movement, by "staking" the left side of the receiver.  Basically, using the center punch, you punch on the threaded part of the receiver, right by the barrel extension locating pin..... to *swell* the aluminum of the receiver outward, thereby "tightening" up the space, and limiting leftward rotational movement of the barrel in the receiver.  

I stake it about 1/16" from the edge, to get the most swell.  This will slightly damage the threaded area right there, but not to worry, as you thread on the steel barrel nut it will clean those threads.  I spin the barrel nut on and off a few times after staking, without the barrel in the receiver, to clean up the threads first.