I have a barrel that needs "re-indexing" so I'm told by Bushmaster. How do they accomplish this? Is the modification performed on the front sight gas block? or do they pull the alignment pin, weld the hole and drill a new pin hole?

I assume your front sight is canted to one side and you can't get a mechanical zero.

Yes, that is correct

Don't feel alone. The same thing happened to me last year.  I purchased a Bushy dissapator and as soon as I screwed the barrel up I noticed the sight was canted.  At this point in the game since head space is already set, it is the front sight that needs to be properly aligned with the rear.  I would assume if you send the unit back to bushmaster they will send you a new barrel, as in their catalog they don't install front sight housings.  I confirmed this with a telephone call to them. On mine I ended up pulling the canted front sight housing and replacing it with a Armalite unit.  The Armalite unit is held on by two allen head bolts and basically clamps on the barrel.  You can set your windage with it and then get a true mechanical zero. If I were you I would just send the unit back to Bushy. They have good customer service.

As long as the barrel extension and front sight are indexed with each other, it's simple. If not, then the barrel itself needs to re-indexed to fix the problem.


To re-index a barrel in the upper.

Pull the hand guards.
Remove the gas tube.
Install the upper in a upper block vise and clamp into vise.
Remove the barrel nut to allow the barrel to slightly rotate.
Drop a sight plain tool/bar in the up and check the location of the front sight to upper, then zero the front sight to the handle.
Re-install the barrel nut, then re-check the index again.
Re-install the tube and hand guards, your done.

I have left out a lot of information and detail, but you get the jest of the idea.  It takes me 4 min to re-index a barrel, but then again, I can build up a rifle from a box of parts in less than 10 min.

Here is the repair manual to study before you attempt the re-index.

old.ar15.com/books/TM9-1005-319-23.pdf

Your looking at about $80 in tools or more depending on your current tool inventory.  If your new to the AR-15, then your best bet is to have Bushmaster just do the work. Most people tend to over rotate the barrel and need to strip it down serval time to get it correctly index to zero.

P.S. It is common on production rifle to have the barrel slightly off index. I believe that if the rear site needs to be adjusted less than 12 clicks from center, the upper is indexed within mil-spec.  
It's us perfectionist that have to have the rear sight dead center at zero to be happy.

Hope this helps.

I also have the same problem with my Colt. I have removed my barrel and cannot see how to rotate the barrel in the barrel extension. Is the vertical alignment pin, pinning the ext. to the barrel?? Are you actually indexing the barrel in the extension? Which is what I would assume.

I would like to get this resolved myself. I need to learn more about the inner workings of my weapon. Do not like the idea of sending her away to the doctor for an extended stay.

The alignment pin is a precise fit, it does not allow the barrel to move. Thats why I was wondering if they weld that hole and drill a new one to re-index.

Most of the problem is because the alignment pin on the barrel extension and the actual placement of the front sight are not in line.  This is because when the front sight housing is drill and pinned to the barrel it is not in line with the pin.  This is why I think Armalite has a better mouse trap by using the clamp on front sight housing.  If you notice all DCM/CMP service rifles have the front sight mounted in this fashion as opposed to drilling and pinning.  This allows them to get a true mechanical zero and drilling and pinning the barrel also affects accuracy. I believe the book is called "The Accurate AR-15' By Barrett Tillman.  In this book they show some ways to cure the aforementioned problem.  One way is to put the barrel in a vice and smack the front sight housing with a mallet in the direction you need it to move. Another method they discuss is driving out the taper pins and reaming the holes out and re-pinning to get proper alignment. The guys that wrote this book are the folks at Accuracy Speaks.