elkbow Wrote:I believe it can definitely affect accuracy, but only in certain circumstances. Consider when you shoot, whether it be off-hand, with shooting sticks, on a bench, your hold changes for each one. If you are shooting off-hand you have one hand on the forearm and one on the grip and finger on the trigger, if you have slop between the upper and lower you can twist the two. Granted, the up and down movement and left and right would be the issues, not so much with rotation, which would be minimal. Now another point, the scope is mount to the upper, therefore it reduces the issues with the slop, where I believe it comes in to play the most is inconsistency in stability. To me, if you grab the forearm, and hold the grip, see if you can rock it front to back, not up and down or side to side, front to back is where it could have the highest possibility of affecting your accuracy. If you shot from a bench and there was play front to back, conceivably it could rock and move differently for each shot depending on where it settled last.
Now all that being said, I believe it is very minimal. I have a few AR's bedded, however I did not see any noticeable change in accuracy. If you wanted to shoot really long distances where every bit counts, it may be worth the enhancements to the gun, I just haven't seen any benefit from it.
FWIW, it's been tested and found to be a complete non-issue. Upper to lower fit only has the ability to affect accuracy insofar as psychological impact on the shooter. If it bugs you, then it likely won't hurt to tighten it up (though I'd never do it on a "down range" gun!).
Uncle Sam sez:
"As per the TM.
1. With the upper receiver attached to the lower receiver, and the pivot pin and takedown pins in place,
perform the following test:
(a) Apply hand pressure to push the upper receiver as far to one side as possible.
(b) Attempt to insert a 0.020 inch thickness gage between the pivot pin lugs or the upper and lower
receivers.
© If the thickness gage penetrates to the pivot pin at all accessible locations, repair by replacement of
the upper receiver (see (b) below) or replacement of rifle is required.
2. If the rifle fails the above test, remove the upper receiver and install a "NEW" upper receiver and perform
the test again.
3. If the rifle now passes the above test, it shall be considered serviceable and continue in use.
4. If the rifle fails the test with a new upper receiver, this failure shall be considered a shortcoming. This
shortcoming requires action to obtain a replacement rifle. Once a replacement has been received,
evacuate the original rifle to depot for overhaul.
3-68"
If someone has an upper that is that loose, then it needs to be corrected/replaced. Otherwise, rock on.
If anyone out there has a copy or a link to the Armalite paper on receiver fit please post it as my google foo is failing me tonight.
ETA: Here is one of them...http://www.armalite.com/images/Tech%20No...htness.pdf.
I'm pretty sure AMU tested it also.