This has probably been covered before. I know some folks like a tight fitting upper and lower. But does it technically make for a more accurate rifle? Has anyone ever tightened up a loose upper/lower and seen an increase in accuracy?

Mick

This is an interesting issue - I've always heard that upper to lower fit does not affect accuracy.  But to me, this is the same issue as bedding a bolt action rifle.  You're trying to eliminate the slop in the action to stock fit when you bed a bolt-action rifle and it supposedly increases accuracy a great deal.  In both cases the sights/scope are attached to the action/upper receiver so that sight is consistantly aligned with the bore whether the stock is bedded or the upper to lower fit is tight.  

What am I missing here?

While there are those that claim use of the "Accuwedge" to tighten the fit between the upper and lower on a AR15, is beneficial to the extreme.

Then there's those, who like above say NOT!

In testing I have proven with my LR CR6724 rifle there is only a "slight" reduction in group sizes, somewhere in the neighborhood of 1/32nd of an inch, and then again perhaps I just did a better job of holding harder/tighter on the 3-5 shot groups involved.

I use them, but NOT for the great reduction in groups sizes, simply because I like a tight fitting AR.

They work fine for that purpose, but their not gonna turn a 2 MOA AR into a 0.25 MOA tack-driver.

Mike

shooting.
1.  put the sights on target and pull the trigger.
2.  the bullet travels down and leaves the barrel.

if the gun moves between the time steps one and two occur accuracy is lost.

a loose fitting upper and lower can cause this to happen.  

My eye placement is dependent upon cheekweld to the stock.  If the upper can move and the sights are mounted to the upper and my cheek is on the stock which is attached to the lower that means I will not be able to get the exact same sight picture every time if the two are moving relative to one another.

So IMHO, yes it affects accurancy, though how much I cannot say.  I figure an accuwedge cannot hurt anything on a loose lower...

Does it affect the ultimate accuracy of your rifle, no; but it can affect how well you shoot your rifle.

JimmyT's point is somewhat astray I feel, bolt action rifles are a different issue as bedding can affect the match between the bolt and the barrel, but in an ar the bolt and barrel are both housed in the upper receiver, the joint between the upper and lower having no affect on the alignment of the two.

Now wait a minute - if the barrel on a bolt-action is free-floated, how would bedding the action change the alignment of the bolt to the chamber?  What you're trying to accomplish with bedding a bolt action is making sure that the action sits the same in the stock shot after shot so that your cheek weld/sight alignment is the same shot after shot.  Or at least that is my understanding.

It has absolutely no effect on accuracy.  Accuracy is determined by barrel and headspace which remain constant in the AR type rifle.  Anything you do prior to squeezing the trigger will effect bullet placement as will jerking the trigger but the bullet has left the barrel long before the recoil has moved the weapon.  Stocks are bedded for two reasons, the first relates to the tendancy of wood to absorb moisture, the second relates to the bolt locking in the receiver rather than the barrel extension.  

Well the wood stock theory holds no weight because the top shooters use fiberglass, kevlar, and other composite stocks that are bedded.  

I don't understand your other theory either because bedding a bolt-action receiver doesn't change the way the bolt locks up in the receiver.  It only ensures that there's no movement of the action within the stock.  Truing the action is what ensures the bolt locks up squarely and consistently.  

Barrel and headspacing are only two factors affecting accuracy.  A consistent cheek weld and sight picture are also very important, especially with optics as an inconsistent sight picture can cause a change in POA, resulting in a change in POI.  I think that's the issue bedding helps resolve.  

I tested this about 15 years ago with an Olympic MultiMatch chambered in 6PPC.  The upper and lower were extremely loose fitting, but it still shot very consistent 3/4" groups.  I probably shot 200 groups through that gun during load development work, so this was not just a "couple" of groups.  Then I picked up one of those JP expanding rear pins and the gun was locked up tight.  I was sure the group sizes would decrease, but they did not.  Repeating the same loads, netted the same average group size.  No improvement.  This test was only conducted on this one particular gun, so results may vary with your own gun.

Tony Rumore
Tromix Corp

Barrel harmonics!!!!!

To achieve the best accuracy in regards to the barrel/receiver/stock fit, the barrel needs to resonate the same from shot to shot or the end whip of the barrel will be at a different location in it's elongated circle movement when the bullet exits the muzzle.

A loose upper to lower, or a receiver to stock, is more likely to create a different input/absorption of the barrel harmonics from shot to shot.  This is the reason that bolt rifles actions are glass bedded, to lessen the effects of the receiver moving in the stock and affecting the true barrel harmonics/movement.

On full blow space guns, setscrews are used to tighten the upper to the lower for a rock solid fit between the two. Granted that installing setscrews on a service rifle may not be a option, but installing a Accuwedge and shimming the front take down pin/block, or using a wedge pins is a option that is easily achieved in only a few minutes.

If you have a rifle that is producing groups in the 1-1/2" to 2" MOA, tighten the upper/lower fit will not turn the rifle into a Sub MOA shooter, but it will slightly tighten up the groups, due to the barrel being more consistent during the barrel firing/movement harmonics.   And for the record, the only thing moving on the rifle until the bullet leaves the barrel is the barrel it’s self.  The bolt doesn’t start moving until after the bullet leaves the muzzle, which brings use back to barrel harmonics, and how constant it is from shot to shot.

Where the heck are you supposed to put those accuwedges anyway? Hmm, I think I may be setting up a joke answer....

[img]http://www.hsgear.biz/SbSLib/thumbnailinator.asp?txtImagePath=85-ACCUWEDGE.jpg[/img]

Mike