AR15 Upper and lower fit affect accuracy?

[entropy]

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.

Don't have a rifle rest? (Lead Sled, etc?) The stock on a rifle is merely an operator interface. Unlimited class benchrest rifles sometime have no stock at all in the traditional sense, and are very accurate.

 

[MistWolf]

Thats not an answer at all.

Actually, it is. The answer is, the slop between the upper and lower does not affect precision or accuracy. It does not change POI or POA

Thats just saying the gun is equally imprecise when taken apart and put back together.

This is true. But the imprecision does not come from the play between the upper and lower. If it did, shooters could swap around uppers and lowers until they found a combination that gives best precision. But that doesn't happen.

The receiver on a bolt action rifle also locks the barrel, receiver and sights together in a constant relationship.

That's because stock bolts hold the barreled action to the stock through tension which stretches the screws and bends the action. This introducing a springiness which affects harmonics. When you remove the action from the stock and re-torque the stock screws, the harmonics are changed, thus changing POI/

There is no springiness between the upper and lower to affect harmonics. Thus, there is no change in POI from removing the upper from the lower and no affect on precision from the play.

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.

It doesn't. It only changes how we look at the upper receiver. The upper receiver is just there to hold parts in place, not to contain pressure.

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.

How you hold an AR does have an affect on practical precision, but not mechanical precision, same as any other rifle. Just as any rifle, there is a right way and a wrong way to set an AR in a rest.

Physics says there is an equal and opposite reaction to everything and physics has no delay.

This is not true. The bullet is accelerated down the barrel and the rifle is accelerated back towards the shooter. Acceleration is a change of speed with a change of time, or a change of direction with a change of time. Since a key part of acceleration is a change of time, that tells us acceleration cannot be instantaneous. If it is not instantaneous, there is indeed a delay. Whether or not the delay is enough to prevent the rifle from moving before it reacts to recoil forces is another discussion. However, what we do know is that the shock waves from igniting the gunpowder travel through the steel of the rifle at the speed of sound causing the barrel to flex. I don't know the numbers, but the speed of sound in steel is much faster than the speed of the bullet travelling through the bore. That means barrel whip starts before the bullet exits teh muzzle.

Have you tried?

I have talked to shooters who have. I've also talked to shooters who have successfully built precise ARs. Knowledgeable precision AR shooters, almost to a man, agree that it's about the barrel and the ammo. Improvements have been seen by matching the headspacing of the bolt to the barrel. Some claim that a tight fit between the upper and barrel extension also helps. I've yet to talk to anyone that has seen a measurable difference from bedding of fitting the upper to the lower. Most who still do tell me "Well, you never know."

The precision AR I built uses a Walther-Lothar 20" barrel with a matched bolt and an VLTOR upper. It shoots tight little three shot groups at 100 yards with quality ammo. By tight, I mean the shots touch. But, three shot groups don't tell us very much.

I had a Colt Competition HBar with a wedgelok. It made no difference to precision whether it was installed or not. All it did was make pulling the take down pins harder. It honestly did no better at removing slop than a loaded mag.

It's your rifle, it's your money. But if you want methods proven to improve precision in an AR, use a quality match barrel and have the bolt headspaced matched to the barrel. Use an upper receiver that's stiffer than normal and has a tight receptacle for the barrel extension. Use quality match ammo. You already have ammo well in hand. If don't use a quality barrel, headspace match the bolt to the barrel and use a good upper, what little if any help bedding the upper to the lower would get lost in the noise.

 

[someguy2800]

Don't have a rifle rest? (Lead Sled, etc?) The stock on a rifle is merely an operator interface. Unlimited class benchrest rifles sometime have no stock at all in the traditional sense, and are very accurate.

Of course I have a rifle rest but but none of them have provisions to hold a stockless rifle or a lowerless AR15. The inherent accuracy of a barreled reciever is of no concequence to me if the rifle as a package does not allow me to shoot it accurately.

I have talked to shooters who have. I've also talked to shooters who have successfully built precise ARs. Knowledgeable precision AR shooters, almost to a man, agree that it's about the barrel and the ammo. Improvements have been seen by matching the headspacing of the bolt to the barrel. Some claim that a tight fit between the upper and barrel extension also helps. I've yet to talk to anyone that has seen a measurable difference from bedding of fitting the upper to the lower. Most who still do tell me "Well, you never know."

I had a Colt Competition HBar with a wedgelok. It made no difference to precision whether it was installed or not. All it did was make pulling the take down pins harder. It honestly did no better at removing slop than a loaded mag.

We are just talking past each other on alot of this and its unproductive so I'm just going to trim this down. This above is the feedback I was looking for thank you for that. All have been able to find reading is everyone just repeats what they are told or they say I tried the same upper on three different lowers and couldn't tell the difference. Actual tested results are much more interesting than theories. I'm going to do a bit of testing of my own when the weather improves so if I can't find a noticeable improvement in accuracy or large point of impact change depending on how the pins are tensioned then I will put this to bed, at least for myself. As far as money it would be free because I would be the one doing it.

 

[greyling22]

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! 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.

could you clarify this one for me. so the lower is the firearm because the fire control group is there, but then in the ruger 22 pistols the upper is the firearm. In the sig 320 the trigger pack is the firearm, but in the 10/22 the trigger pack is not and the receiver is.


PS: and yes, I am aware I asked "why" and "atf" in the same thought, which is the first step toward madness.

 

[entropy]

No, I don't know why different guns have different parts considered the firearm, nor do I particularly care.

 

[Walkalong]

My question would be do you think you can shoot the difference and do you need such a high degree of accuracy that a very tiny loss of accuracy would be a problem.

For myself I quit worrying about it a long time ago. We had a fellow with an AR come to a registered Benchrest match one time. We let him shoot along side us all the first day. He shot some very nice groups, with one really nice group. He did not do anything to tighten up the upper/lower, he simply had the upper smithed by someone who knew what they were doing. We were all duly impressed. This was off of a nice rest and rear bag of course, and he wasn't taking it completely apart, but he did pull the rear pin and clean the rifle a couple of times. If the POI changed from that he didn't mention it.

 

[vtsteve]

I'm sure the accuwedge thingy works just fine, but here's a little trick I learned to take the slop completely out of an upper/lower fit for next to nothing. Find a small o-ring or rubber band that's just big enough to fit around the front "nub" on the upper receiver that it pivots on during takedown. Separate the receivers, put the rubber band on and put them back together (you'll feel a little resistance as you pivot it back together to pop in the back pin). That's it. The rubber squishes and takes up all the empty space and locks it up tight. If the rubber band wears out after a few dozen open/close cycles just replace it.

 

[someguy2800]

My question would be do you think you can shoot the difference and do you need such a high degree of accuracy that a very tiny loss of accuracy would be a problem.

For myself I quit worrying about it a long time ago. We had a fellow with an AR come to a registered Benchrest match one time. We let him shoot along side us all the first day. He shot some very nice groups, with one really nice group. He did not do anything to tighten up the upper/lower, he simply had the upper smithed by someone who knew what they were doing. We were all duly impressed. This was off of a nice rest and rear bag of course, and he wasn't taking it completely apart, but he did pull the rear pin and clean the rifle a couple of times. If the POI changed from that he didn't mention it.

Well if my goal was to build the most accurate rifle possible I wouldn't have built an AR to begin with, but I am curious and this is my hobby, and spending time working on something is not a detraction to me. I built this gun just to be a fun plinker but I'm a sucker for accuracy so now its morphing into a light varmint gun. I seek out things like this to do because its enjoyable to me. If there is a change in group size of .1 to .2 moa I will be able to see it within a few groups.

This was the best 5 shot group from Sunday with this gun. It measures 3/4" center to center. I ran out of targets so I cranked up the scope a minute and was shooting at the black sharpie dot. I was shooting at my father in laws place so I just had the gun resting on a rolled up jacket in the front and a towel in the back that was in my suburban. The rest of the groups with this powder bullet combo were 7/8" to 1-1/8". I just started load development so I've not started shooting larger groups yet. I'm also shooting unprepped winchester brass so I'm sure I can find some more with brass prep.

 

[Varminterror]

This is not true. The bullet is accelerated down the barrel and the rifle is accelerated back towards the shooter. Acceleration is a change of speed with a change of time, or a change of direction with a change of time. Since a key part of acceleration is a change of time, that tells us acceleration cannot be instantaneous

This is absolute garbage. Terrible misrepresentation of science being used to mislead here. Usually you do far better on your math.

If force is exerted on the bullet forward, the same force is exerted rearward on the rifle, and momentum is conserved. The fact the rifle is far heavier than the bullet means the rifle is accelerated much less than the bullet - if you do the free body diagram for the two, F = M1A1 = M2A2 with 1 being the bullet and 2 being the rifle. If a bullet is 150grns, the rifle 9lbs (63,000grn), then the ratio of M1/M2 is 0.00238, therefore the ratio of acceleration is then the inverse. F=M1A1=M2A2, further then M1/M2 = A2/A1. So the bullet accelerates 420 times faster than the rifle.

The acceleration of the bullet, and therefore the rifle, are not constant, however, anyone with Quickload can run the velocity curve, run the integral and get the instantaneous acceleration of the bullet, and therefore the rifle. It's remarkable how simple rocket science really is, in that way.

But at midnight with my give-a-**** pretty low, let's pretend the acceleration were constant, because even this generalization will illustrate why you think you can get away with saying it is delayed. Say you're talking about a bullet starting at zero and exiting at 2900fps from a 24" barrel. Average speed, with constant acceleration, is then 1450fps, and in a 2ft barrel, we're talking 1.38 milliseconds of normalized dwell time. So going from zero to 2900fps in 1.38 milliseconds is 2,102,973 feet per second per second in acceleration. So let's look the other way... F = MA, so F=M1A1=M2A2, meaning the force pushing the bullet from the rifle (neglecting heat, friction, sound, vibration, blah blah), so M1 = 150/7000, A1 = 2,102,973, and M2 = 9lbs, so A2 then is 5,007 feet per second per second... A bit less impressive. The resultant velocity due to acceleration, starting with zero velocity, is V=AT, so the velocity of the rifle when the bullet leaves the bore is 6.9fps. It's displacement, X = 1/2AT^2, will be 0.0048ft, or 0.057 inches... Not much displacement, considering the bullet will have traveled 24" in the same time, but displacement nonetheless - and of course, that's considering the rifle free in space, without the inertial mass of the shooter's shoulder against the stock...

It's easy to say the rifle's motion is delayed, however, it is not. The acceleration is happening in both directions exactly in congress, however, one is accelerating FAR less than the other.

 

[MistWolf]

All I was trying to explain is that it's not instantaneous. It takes time to get things to start moving. That's why there is a pressure peak at the start.

 

[M1Jeff]

As has been stated above, the precision in an AR15 is in the upper assembly. As Derrick Martin, a former match armorer for the All Guard Service Rifle Team stated in his book on AR accuracy, "Float a good barrel and put a good trigger in it." After extensive testing, his conclusion was pick a good barrel, chamber it properly for the ammo you intend to use, free float the barrel and make sure the gas tube isn't binding in the gas key. That's the precision part of it.

The lower is not a stressed part of an AR15. It has been referred to as nothing but a trigger hanger. I know, I'm oversimplifying.

So for shooting it accurately, which I take to mean POA is POI, how does the lower fit factor in?

For most shooting applications, not much if at all. But we are assuming just a bit of slop.

For sling position shooting, any slop is torqued away when we shoulder the rifle. For bipod shooters, I imagine it's much the same when you preload the bipod legs.

How much slop is tolerable? In my experience, when there is so much play that even slung in the receivers are shifting around, it's time to replace the receiver halves. I experienced this in a particular match at Camp Perry one year. It was shot using M16s the Army Marksmanship Unit brought up from Benning. Even so, I managed to score 97/100 with it in slow prone.

Now this is only my opinion, me not being a benchrest shooter: for BR testing, I would want the receivers as slop free as I could get them. Why? Because my understanding is that the rife is not snugged tightly into the body therefore torquing the slop out as in position shooting.