AR15 Upper and lower fit affect accuracy?

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someguy2800

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I read all the time from people that the slop between the upper and lower on an AR has no effect on accuracy and was also told this by the local AR builder. This just make no sense at all to me how this can be when you look at the difference it can make glass bedding a bolt action rifle. It would seam to me any play between the upper and lower would be like leaving the action screws loose on a bolt rifle which I know is not going to do you any accuracy favors. Could the difference between a 1.5 moa AR15 and a 1 moa AR15 be the upper and lower tightness?

What do the people building match grade AR's for competitive shooting do? Do they not do anything to tighten up the fit between the two?
 
There's a gizmo called an Accuwedge that tightens up the fit. I have them installed on 2 of mine. I'm not sure it does anything, but it sure can't hurt.
 
The upper and upper alone dictates the mechanical accuracy potential of the AR. However, if there is slop, it may affect the shooters ability to utilize that accuracy due to two of the 3 points of contact the rifle has with it's platform not being rigidly connected to the upper.
 
You could completely eliminate the lower, and design a device to hit the firing pin and it wouldn't affect the accuracy. Everything that could affect the accuracy is contained in the upper; sights, barrel to receiver interface, chamber, rifling, bore, etc. Of course, you would then have a single-shot, but it would still be accurate. Hey, I just invented the unlimited class bench AR! :thumbup: Needless to say, you'd have to add some sort of bolt stop, or that could be part of the competition, add in the bolt's flight to the score somehow.;)

Mach IV added in while I was typing the sole reason it could affect accuracy.

The lower provides the trigger group and the mechanisms that 'run the gun' -safety, recoil spring and buffer, magazine well, magazine, and magazine release, but technically speaking it is not necessary to fire the gun. The reason the ATF designates the lower the gun on an AR is that since the Fire control Group is in it, and definition of the type of weapon it is is determinted by type of FCG, the lower is the gun.
 
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OP: the reason the upper and lower fit on an AR doesn’t affect accuracy while stock fit on a bolt action does is because of the design differences between those two types of rifle.

On an AR the barrel, forend, action, and all sights (iron or optic) are all contained in the upper receiver. So the fit of the upper to the lower has no affect on the mechanical accuracy of the firearm.

But that’s not the case on a bolt action rifle: the forend contacts the action (and in some cases the barrel), so the fit of the stock to the rest of the rifle can affect accuracy.
 
If you take a bolt action rifle with a free floated forend the barreled action is basically the upper and the and the stock is the lower. So you could also say the same thing that all the accuracy related components are all located in the barreled action, but we know the fitment and tension between the two have a huge impact on accuracy with that. It still doesn't make any sense to me but I guess it is what it is.
 
I don't know that the AccuWedge helps accuracy, but I hate shooting a sloppy loose
rifle. So if it doesn't actually help, it surely makes me feel better. It tightens the rifle up,
and they cost next to nothing.
 
but we know the fitment and tension between the two have a huge impact on accuracy with that. It still doesn't make any sense to me but I guess it is what it is.
That's just it the tension and fitment between the stock and action doesn't have a "huge difference" in accuracy, as long as the action is attached to the stock in a manner that keeps the barrel "floating" accuracy will be good.
 
someguy2800 said:
If you take a bolt action rifle with a free floated forend the barreled action is basically the upper and the and the stock is the lower.
No, not exactly. In the case of a free-floated bolt-action rifle, the forend is attached to the action, and the stock is directly attached to the forend. That's not the case on an AR: the stock and the lower receiver is entirely separate from the forend.
 
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That's just it the tension and fitment between the stock and action doesn't have a "huge difference" in accuracy, as long as the action is attached to the stock in a manner that keeps the barrel "floating" accuracy will be good.

That's not been my experience in the rifles I have bedded. I've had two rifles that were totally floated ahead of the recoil lug but there was movement of the action in the stock from side to side, and accuracy improved significantly when glass bedded.

I don't know that the AccuWedge helps accuracy, but I hate shooting a sloppy loose
rifle. So if it doesn't actually help, it surely makes me feel better. It tightens the rifle up,
and they cost next to nothing.

That's exactly why I put one in mind. The rattle drove me nuts.

No, not exactly. In the case of a free-floated bolt-action rifle, the forend is attached to the action, and the stock is directly attached to the forend. That's not the case on an AR: the stock and the lower receiver is entirely separate from the forend.

If it's free floated the forend doesn't interact with the action at all. It's just a place to hold the gun. If the forend being attached to the receiver vs the stock is the only difference then wouldn't it make a difference on an AR if you rested the front of the lower instead of the forend?
 
No, not exactly. In the case of a free-floated bolt-action rifle, the forend is attached to the action, and the stock is directly attached to the forend. That's not the case on an AR: the stock and the lower receiver is entirely separate from the forend.

I'm not sure I follow. Let's imagine two rifles:

1)An AR upper with no handguard - just the upper and barrel + a lower
2)A bolt action (just the action and barrel) + a stock (with the fore end sawed off if you like).

IIUC the theory behind bedding the action for #2 is that if you don't, the action may end up in a slightly different place relative to the stock each time, and that could affect the harmonics when you fire. For example, if the recoil lug is right back against the stock recess for one shot, but bounces 0.005 inches forward before the next one. I have a well worn Garand where you can easily move the action .020 relative to the stock. ISTM that same kind of slop would equally affect an AR upper, if it existed.

Having said that, the nature of how the takedown pins work keeps there from having much movement between an AR upper and lower, unless the pin holes in the upper are waaaaay big. The distances between pins/holes just have to match pretty closely between the upper and lower or things won't work, so I don't think the worst AR upper/lower fit is going to allow more than a thou or two of relative movement.
 
No. We proved this putting a MK12 upper that grouped sub MOA on different lowers- some were tight, some sloppy. Consistent groups. If the slop bothers you, throw in a accu wedge, They are cheap.
 
someguy2800 said:
If it's free floated the forend doesn't interact with the action at all.
I'm no expert on bolt-actions, but isn't the stock attached to the receiver? And the forend is integral with the stock. So in that case, pressure on the forend would directly effect the receiver, which is directly attached to the barrel.

pintler said:
I'm not sure I follow.
I don't think I've been explaining my point very well. My facts might be wrong about the effects on accuracy that I'm describing, but my point is that the OP's description equating a bolt-action's stock to an AR's lower receiver is incorrect simply because the stock and forend on a bolt-action are all part of one piece that is attached to the action, whereas on an AR that's not the case.

Please correct me if I'm wrong (I'm definitely no expert on this subject), but on a free-floated bolt-action the stock is attached starting at the forward part of the receiver, right? And since the forend is integral with the stock, that means when pressure is applied to the forend it can change vibration harmonics with the forward part of the receiver. And since the action is directly connected to the stock, this can therefore have small but measurable effects on accuracy.

On an AR that pressure works in a similar way: Even on a free-float barrel, pressure on the forend can change the harmonics on the front of the upper receiver. Since that is the part that touches the barrel, that pressure on the forend can also have small but measurable effects on accuracy.

However, on an AR the forend is part of the upper receiver, not the lower receiver, so upper/lower fit has nothing to do with the fit between the forend and the action. But on a bolt-action rifle the stock/action fit is directly related to how the forend interacts with the action.
 
Guys have been mitigating upper-lower slop for generations. It DOES affect consistency, but throwing a little torque on the pistol grip - CONSISTENT torque through regular practice - helps ensure the upper and lower are situated the same in every shot. Whether it's the new age precision AR instructors or the old school Highpower coaching course, this is common fodder.

Precision AR builders have been matching lowers, shimming, tensioning, blocking, wedging, bedding... for a long, long time. The smith I apprenticed under when I was still in high school taught me a handful of these methods ~20yrs ago, and he wasn't the pioneering type, he'd been taught them by someone else long before he passed them to me.
 
If you have an upper and lower that are both within spec the amount of wobble is so small that it won't make a difference. If you have enough wobble to have it affect accuracy then it's either not made within spec or it's worn out. There are a lot of things that will affect the accuracy more than a little wiggle in the recievers. Like barrel profile, what kind of handguard, even the way the bolt slams the rounds into the chamber.
 
I read all the time from people that the slop between the upper and lower on an AR has no effect on accuracy...
How much change in POI are you getting each time you separate the upper from the lower?

The best way to take out the play is to insert a loaded magazine in the well.
 
My thinking is the natural tension with which it is fired would mitigate nearly all possibility of the lower affecting accuracy, much as Varminterror points out.

The sole component contained in the lower that might have a potential to change things is the magazine, and only at an extreme where a round could be damaged by loading angle. I don't claim to know, but I suppose at that point, and only if possible, it could introduce the same amount of inaccuracy as any other magazine fed rifle. Or I could be wrong entirely.
 
The upper and upper alone dictates the mechanical accuracy potential of the AR. However, if there is slop, it may affect the shooters ability to utilize that accuracy due to two of the 3 points of contact the rifle has with it's platform not being rigidly connected to the upper.

^ Exactly what he said.

The trigger, stock, lower, and other parts all serve only to aid the shooter in delivering an accurate shot. The inherent accuracy of the gun comes from the upper.
 
Unfortunately I can't shoot an AR without a lower just as I cannot shoot a bolt action rifle without a stock. When the weather clears up here I'll have to do a test. I'm curious if shooting my AR while rested on the stock and forend which puts tension on the takedown pins produces a different group size and point of impact if I rest the gun in the mag well instead which would leave the takedown pins untensioned. My AR does have a rather loose upper and lower fit but does shoot about .75 to 1 moa off the bench.
 
How much change in POI are you getting each time you separate the upper from the lower?
 
How much change in POI are you getting each time you separate the upper from the lower?

None that I have been able to notice.

I'm moreso wondering how much the upper and lower movement affects POI in different shooting positions and if reducing the amount of play between the two can affect accuracy. It seams the group consensus of those that have actually tried it is that it makes little to no difference. I'm willing to accept that but I don't really like accepting something blindly without doing some testing of my own to confirm. I've disproved plenty of my own conceptions of what I believed to be fact through testing.

Where this is all coming from is that I had a Savage BVSS that I rebarelled a few years ago with a criterion 223 barrel. I was never able to get the rifle to really wow me with accuracy. About .75 to 1 moa for 10 shots was the absolute best I could get out of it, though it would shoot nearly any load into that group size. A bvss comes factory with a boyds stock and stainless bedding pillars. There was still enough room between the reciever and stock on that gun that if you set the butt on the ground and pushed and pulled on the barrel that you could still move the whole barreled action side to side in the stock by a small amount even with the action screws as tight as one could get them. You could not with any force applied make the barrel come close to touching the forend. After I glass bedded the action into the stock you could not move the action at all relative to the stock and it would then shoot .5 to .75 moa with any load and bullet. I had spent a great deal of load development to try to break through that .75 moa barrier on that gun but eventually found that the mechanical connection between the stock and action was making that impossible.

The AR I'm working on now is in about the same just under 1 moa category with the right loads and bullets, though as of yet I have only tested a few load combinations. But if there is the potential to reduce that by even .1 moa then it will be a worthwhile and enjoyable exersize to see what I can do with the upper and lower fitment. Obviously an AR by design cannot place the upper and lower in tension to one another but I'm thinking of reaming the pin holes in the upper to press in a tight tollerance bushing in the front and epoxying one in the back to get the spacing perfect, and possibly working on some kind of standoff to eliminate as much side to side movement as possible. This is just the kind of stuff I like to mess around with.
 
What if the improvement in your Savage wasn't because of making the connection more rigid, but because you added a dampening mass (the bedding) that changed the harmonics of the barrel?
 
None that I have been able to notice.
There is your answer. The play between the upper and lower doesn't matter. Changing the upper to another lower also does not change the POI.

The AR upper locks the barrel assembly, receiver and sights together in a constant relationship to one another. The reality is, the part of the AR we commonly call the "upper receiver" is analogous to the stock of a bolt action rifle. The actual receiver of an AR is the what we call the barrel extension. It's what the bolt locks into and holds the pressure of firing.

No stress at all is placed on the upper when it's dropped onto the lower. There is no contact between the lower and the barrel extension or the barrel. There is no contact with the bolt when the bolt is in battery. Because the lower simply floats on top the lower, because there are no fasteners that need to be torqued, the contact between the two halves is without any kind of force or stress to distort the upper.

The play between the upper and lower has no more affect on mechanical precision. (What we are talking about is actually precision, how close bullets group. Accuracy is how close we hit to the intended target.)

I'm moreso wondering how much the upper and lower movement affects POI in different shooting positions and if reducing the amount of play between the two can affect accuracy. It seams the group consensus of those that have actually tried it is that it makes little to no difference. I'm willing to accept that but I don't really like accepting something blindly without doing some testing of my own to confirm. I've disproved plenty of my own conceptions of what I believed to be fact through testing.
What many people don't realize is that when a loaded magazine is inserted in the well, the top cartridge pushes up on the lower and takes out the slop just as effectively as any polymer Wedgelok. The loaded mag also has the advantage of not hardening from time and chemicals to fall apart inside the action. Load up a magazine and test it for yourself.


Where this is all coming from is that I had a Savage BVSS that I rebarelled a few years ago with a criterion 223 barrel. I was never able to get the rifle to really wow me with accuracy. About .75 to 1 moa for 10 shots was the absolute best I could get out of it, though it would shoot nearly any load into that group size....After I glass bedded the action into the stock you could not move the action at all relative to the stock and it would then shoot .5 to .75 moa with any load and bullet.
Any rifle that consistently shoots ten shot groups to MOA is awesome. Any rifle that consistently shoots ten shot MOA groups with any bullet is amazing. A rifle that consistently shoots ten shot groups to .75 MOA with any kind bullet after a simple bedding job is legendary!

The AR I'm working on now is in about the same just under 1 moa category with the right loads and bullets, though as of yet I have only tested a few load combinations. But if there is the potential to reduce that by even .1 moa then it will be a worthwhile and enjoyable exersize to see what I can do with the upper and lower fitment. Obviously an AR by design cannot place the upper and lower in tension to one another but I'm thinking of reaming the pin holes in the upper to press in a tight tollerance bushing in the front and epoxying one in the back to get the spacing perfect, and possibly working on some kind of standoff to eliminate as much side to side movement as possible. This is just the kind of stuff I like to mess around with.
Don't bother reaming the pin holes or trying to bed the upper to the lower. That stuff won't help. If you are serious about getting the best precision possible from your AR, start with a quality match barrel with a matched bolt. Mount it in either a forged VLTOR upper or the newest generation BCM forged upper. Both are designed to be stiffer than other uppers and offer a nice tight fit between the barrel extension and the upper. The next item is quality ammo, but you've already got that covered.
 
There is your answer. The play between the upper and lower doesn't matter. Changing the upper to another lower also does not change the POI.

The AR upper locks the barrel assembly, receiver and sights together in a constant relationship to one another. The reality is, the part of the AR we commonly call the "upper receiver" is analogous to the stock of a bolt action rifle. The actual receiver of an AR is the what we call the barrel extension. It's what the bolt locks into and holds the pressure of firing.

No stress at all is placed on the upper when it's dropped onto the lower. There is no contact between the lower and the barrel extension or the barrel. There is no contact with the bolt when the bolt is in battery. Because the lower simply floats on top the lower, because there are no fasteners that need to be torqued, the contact between the two halves is without any kind of force or stress to distort the upper.

The play between the upper and lower has no more affect on mechanical precision. (What we are talking about is actually precision, how close bullets group. Accuracy is how close we hit to the intended target.)

Thats not an answer at all. Thats just saying the gun is equally imprecise when taken apart and put back together. The receiver on a bolt action rifle also locks the barrel, receiver and sights together in a constant relationship. I just don't see how moving the locking lugs from the receiver to the barrel extension changes how the gun recoils as an assembly. As the bullet is moving down the barrel any inconsistency in how the gun is allowed to move is going to contribute to inaccuracy. That includes the person sitting behind the stock. It may be that the just the tension on the pins from the gun resting on the forend is enough to preload the assembly so that it recoils in the same manor shot to shot.

(And before anyone says the bullet is gone before the gun starts to recoil, that is simply not true. Physics says there is an equal and opposite reaction to everything and physics has no delay. The movement during bullet dwell is very small but it is there. If you do the math on it, changing the relationship of the muzzle to breech end of a 20" barrel by one thousandsth of an inch is a .166 moa change. .010" of movement is 1.66 moa)

Any rifle that consistently shoots ten shot groups to MOA is awesome. Any rifle that consistently shoots ten shot MOA groups with any bullet is amazing. A rifle that consistently shoots ten shot groups to .75 MOA with any kind bullet after a simple bedding job is legendary!

I must admit I'm truly amazed any AR15 can shoot even close to moa. I'm pleased with how mine shoots, but better is always better. When I started this build I thought I would be happy with 2 moa but that didn't last.

Don't bother reaming the pin holes or trying to bed the upper to the lower. That stuff won't help.

Have you tried?
 
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