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Posted: 12/24/2003 11:38:25 AM EDT
I am ordering a Bushmaster carbine upper for my pre-ban PWA lower next week.  I called BM and they said that If I ship them my lower, they will hand-select a tight fitting upper for no extra charge.

During the short conversation, the BM rep said that all their tolerances are vary close from the start.  He didn't think that a hand-selected upper would be too much different.

I can live without my lower for a week or so so sending it away isn't a big deal.  The added shipping costs aren't enormous but it does add to the overall bottom line.

Should I send them my lower or take my chances with what they send to me?  The gun is a "working" gun, not intended for benchrest shooting, so wringing out every last bit of accuracy isn't a major concern.

Link Posted: 12/24/2003 11:41:32 AM EDT
I wouldn't sweat it.  Every new upper/lower combination I've had was tight.  It's going to loosen up with use anyway.
Link Posted: 12/24/2003 9:59:53 PM EDT
Thank you.  I'd rather spend my money on ammunition instead of shipping.
Link Posted: 12/24/2003 11:32:11 PM EDT
[Last Edit: 12/24/2003 11:35:46 PM EDT by ru4freedom]
 If you want a tight fit, the best way I have found is to bed the upper to the lower with Acraglas or a simular bedding compound. I put the compound on the upper.

 This is by far the best way I've found to get the tighest fit possible, and when done right, it's virtually invisible!!

Link Posted: 12/25/2003 11:23:35 PM EDT
Wait now, I just bought a bushy varminter last week.  (okay, well it finally came in a week ago)  It was custom built and they made a few mistakes, so i had to send it back.  One thing I did notice was that the upper "shook" ever so slightly against the lower.  Let's just say if you grabbed the pistol grip and shook it, you could vaguely hear the upper slipping side to side.  I figured this was normal, is this something ELSE they need to fix??

What about the "accuwedge".  Doesn't that tighten the upper up, or is that not a good product?

Link Posted: 12/26/2003 8:26:22 AM EDT
[Last Edit: 12/26/2003 8:26:58 AM EDT by _DR]
My first M16A1 when I was in BCT was so loose it rattled everytime I fired it. But it zeroed just fine, and that did not really affect accuracy. If it bugs you, you can get an accuwedge or tensioning rear takedown pin. It can vary widely but is not a problem or defect.

here is Armalites take on it:

February 7, 2003



ArmaLite® occasionally receives questions from customers about the tightness of the fit between the upper and lower receivers of ArmaLite’s M-15™, AR-10®, and AR-180B™ rifles.

There is an intuitive belief that a tight fit between the upper and lower receivers is essential to accuracy, and that loose fit is bad.  Oddly enough, this is untrue.


The upper and lower receivers of ArmaLite® self-loading rifles, and all other similar rifles based on the AR-10 or AR-15 design, are held together by two pins.  Unlike rifles that are cammed or screwed together and can be tightened, the pins allow for slight movement between the receivers.  

It is possible to selectively fit pins and receivers, or to custom ream the pin holes while the receivers are held together, so that movement is minimized.  Doing so reduces the interchangeability of the receivers, and normal wear results in looseness over time anyway.  ArmaLite® uses the reaming process on its National Match M-15™ rifles.

It is also possible to reduce movement by inserting materials such as paper, rubber, or epoxy somewhere between the receivers and creating a tight fit or bind.  Many commercial customers use a common rubbery device claimed to improve accuracy, and the Army Marksmanship Unit often uses a bit of epoxy.  The AMU Armorers advise that this procedure doesn’t improve accuracy, but it makes the shooter feel more confident about their rifle.

The AMU Armorers are correct.  Formal testing conducted at Rodman Laboratories in the 1970s verified that accuracy wasn’t affected by normal receiver tolerances.  The fact is that the bullet exits the bore before movement of the receivers produces a measurable effect.  

Modern manufacturing processes are unable to produce to dimensions that both allow full interchangeability AND prevent all movement in all cases.  

Conclusion.  ArmaLite® rifles are produced to tighter-than-normal tolerances.  Receivers will normally exhibit a certain amount of looseness.  This is normal and will not affect accuracy.  Shooters may reduce the movement by insertion of a small amount of material between the receivers to improve the feel of the rifle, but it won’t improve accuracy.

Link Posted: 12/26/2003 10:33:08 AM EDT

Great info.  Thanks very much. I won't be sending BM my lower.
Link Posted: 12/26/2003 6:46:35 PM EDT

I tried an accuwedge on some of mine that had a loose fit. It tightens up a bit, but then is a booger to get the rear takedown pin out. I decided I didn't like it. You will get used to it.
Link Posted: 12/26/2003 7:18:03 PM EDT

I had an accu-wedge in my carbine's old configuration.  It did really tighten it up... to a point that I couldn't get it apart without some seroius effort.

I took it out during a recent 3-day AR class. The class instructor even gave me some grief for having it in the gun.  

I'm still very new to AR's and learning more every day.  Thanks for the help.
Link Posted: 12/27/2003 6:09:08 AM EDT
k, here's my 2 cents worth...the barrel and sighting systems are attached to the upper. as long as you have proper alignment of the 'zeroed' sights, the rifle will hit where properly aimed. I can think of no improvement in the upper to lower fit that will enhance accuracy. A nice trigger will help, but all things being equal, assuming a reasonable trigger, the mechanical accuracy resides in the upper alone... flame away...lol
Link Posted: 12/27/2003 6:27:08 AM EDT
[Last Edit: 12/27/2003 6:27:34 AM EDT by _DR]
No, that's is correct. That's basically what the above Armalite article was saying, as proven by testing in the '70's. Any tightening method would primarily be for shooter confidence, and would not affect actual accuracy one way or the other. I have come to accept the receiver looseness as a normal characteristic of the AR/M16 over the years.
Link Posted: 12/27/2003 12:10:14 PM EDT
tiaght, like a tiger!
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