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Posted: 10/7/2004 12:50:13 PM EST
I recently aquired an A1 upper, excellent condition inside and out. However, uppon further examination, I noticed that the FSB is tilted a little to the right. Not perfectly straight up and down. My question is, will this put a damper on accuracy or anything else?
Link Posted: 10/7/2004 12:52:35 PM EST
[Last Edit: 10/7/2004 12:53:47 PM EST by gotm4]
It'll make zeroing it use up all the windage in the rear sight. You would probably want to remove the barrel from the reciever and then reinstall it. And or you may need a new barrel indexing pin in the barrel extension.
Link Posted: 10/7/2004 2:07:50 PM EST
The indexing pin is steel, the receiver is aluminum. I doubt changing the pin will affect it in any way. More than likely the slot in the softer aluminum receiver is dented to one side, probably from the use of barrel vise jaws for barrel installation (note, the front sight base is tilted to the right).

The pin can be shimmed on the left side of the slot.

And when you do all of this, use an action block, not barrel vise jaws.
Link Posted: 10/7/2004 2:31:51 PM EST

Originally Posted By A_Free_Man:
The indexing pin is steel, the receiver is aluminum. I doubt changing the pin will affect it in any way. More than likely the slot in the softer aluminum receiver is dented to one side, probably from the use of barrel vise jaws for barrel installation (note, the front sight base is tilted to the right).

The pin can be shimmed on the left side of the slot.

And when you do all of this, use an action block, not barrel vise jaws.



Damn maybe you should tell Bushmaster, I guess they're clueless.
www.bushmaster.com/faqnew/content_by_cat.asp?contentid=181&catid=100

Question: I have just installed a new upper receiver on my old barrel, and now my rifle shoots way to one side

Answer: This is a problem that took us some time to figure out but it is so simple, we're kicking ourselves. The little index pin that lines up the upper and lower is very soft, so that it can shear off if the barrel is over-torque. This is to prevent damage to the receiver when barreling. This pin will distort slightly before it snaps, and this slight distortion is enough to create the excessive windage you describe. A new pin costs only $1.50 and removing the old pin is as easy as grabbing onto it with vise grips and twisting it up and out. Drive a new one in and try the upper on. The pin may shear when you try to remove it, but they are easy to drill out. Just be sure not to drill into the barrel extension. A new pin will be like having a brand new barrel (as far as the receiver is concerned, it is new) and that windage problem should disappear.
Link Posted: 10/7/2004 3:50:30 PM EST
Hm, interesting. So basically, it's just going to be an issue with zeroing, and the windage adjustment? It's barely noticeable, I noticed it only when looking down at the foregrips and the top part wasn't lined up perfectly. So it's just barely tilted to the right. The rear sight is adjusted to the left side quite a ways. Keep in mind though I have no AR tools or any means of fixing this problem myself right now. Eh, I don't even know if I should mess with it or just leave it like it is. Thanks guys.
Link Posted: 10/7/2004 4:03:55 PM EST
In the trouble shooting forum, there are discussion on this topic... Plus ways of fixing it...

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