I am using an AR 15 in .223 for NRA high power competition.  
I have observed a significant change in the point of impact with my rifle when
using the sling in sitting or prone position compared to from bags.  That is the POI
is significantly lower (like 3" @ 100 yds) when the sling is properly tightened.  My upper receiver was not an
expensive one.
Question:  Are there significant differences in the stiffness of various upper receivers that would
make it worth buying a higher quality / perhaps stiffer upper receiver.

I'm not familiar with the high power competitions. Are free float handguards allowed or are you using the standard GI handguards? That would be your answer. With standard handguards, any change in pressure  will slightly move the barrel and change POI

A free float hand guard will fix the problem, if allowed.   I've seen as much a 6" deviation with a tight sling and standard handguard.  I don't think a different upper will help.

A heavier H-BAR barrel might, but it is still going to deflect if the sling is putting pressure on the barrel.

It's not just deflection.  It's also a change in barrel harmonics.

The Shilen barrel in my match rifle is free floated.

Roger

Interesting. Are you sure there's no contact anywhere?

Most receivers are the same 7075 forged aluminum. There are some billet uppers that address flex points by adding more material (and weight) in certain areas, but I haven't heard of an upper allowing that kind of POI shift.

Is the sling attached to the barrel/front sight, or the handguard?

The sling is connected to the handguard which is tubular and has no
contact with the barrel.

zero your rifle when you are snapped in.

aim off when you are shooting relaxed.

A billet upper may have less flex.
However instead of trying to reinvent the wheel I think I would talk to some of other guys in the sport to see if they have, had, or heard of the issue.

 

How are you're optics mounted?  The receiver/barrel contact point is all of an inch, and is a press-fit - forget deflection.  However, if your optics are attached to your handguard, or potentially influenced by your handguard, then your point of aim could change.  IMHO.

There was a thread posted on TOS, I can't remember the results exactly, but I think I recall the billet being stiffer.

It is true that forging can lead to stronger parts. However the part is not a blanket "stronger", but rather the strength is added depending on the direction of grain flow. The strength added from the slightly denser part is minimal compared to strength that can be gained from optimal grain structure. For some critical components, that additional strength is needed so the dies are designed to flow the metal in a certain way to produce that effect. This can significantly strengthen a part in a certain load path. However for other load paths, it may do little to nothing for strength. It all depends on how the dies are designed.

I would venture to guess that for AR-15 upper receivers, the dies are designed more for ease of manufacturing than to reinforce strength along a certain path. I could be wrong here, but I think it's a reasonable assumption. Therefore I would [also a guess] assume that the thicker billet upper would help with flex more than a forged upper.

Not that any of this matters for the OP because he is shooting NRA High Power and a reinforced billet upper would be illegal for service rifle class.

FWIW OP, this is common for slinging up. I get about the same amount of POI shift when switching from rest to slung up. Pick which position you will shoot the most and zero for that position. If you do both fairly often, then that's a perfect excuse to get another rifle.

If the OP is shooting high power, then he's using irons. Most likely a FSB on the barrel. However you bring a valid point, especially when irons are concerned. A front sight mounted on the handguard is terrible for accuracy when slung up. The handguard is flexing significantly relative to the barrel and if your irons are mounted to it, your POI shift will be huge.

Is your support hand snug between the sling and handguard?



I'm 6'1" and my web is against the swivel which acts like a handstop.

A free float handguard is legal for both match rifle and service rifle competition.

hand guard

I compete in the NRA High Power competition in the target rifle class, any sight.
In that class I am allowed a rifle of almost any configuration with an optic.
I am using an AR that I built with a York Arms lower, Guissele trigger, Shilen match barrel
A2 stock, free floated handguard & a Zeiss 6.5 ~ 16 optic.
I am a current NRA smallbore prone master (99.5%+)  
Apparently, from the information given by many folks that know more about the AR than I do,
the flexing of the upper is essentially normal with the
tension of the sling (I like and use a tight sling).  I have I think several choices.
One is to find a stiffer upper, whether it is forged or billet,
Adjust my optic for each stage of competition to compensate for the flexing.
Hold off as needed.
I've seen a few uppers offered that claim higher strength, such as the VLTOR upper.
any knowledge of these?

Roger

Hmm... perhaps if the receiver face isn't square that could allow the handguard to lever on the barrel nut and move the barrel?
Do you use bags on the bench or in prone? Perhaps the different eye positions could have an effect.
Is the POI shift repeatable if you go from bags to sitting or bags to prone, etc?





 

The POI shift is repeatable,  between bags front & rear on a bench to offhand
to sitting and then prone.  The variation in how much the POI shifts is sling tension.
A sling is not used with the offhand part of a match,  but is used on the sitting
position and prone.  The tension of the sling is different between sitting and prone.
Thus I wind up with three points of impact.
The rifle off bags front & rear will shoot into about 1/2 moa with match ammo.

Roger

What about a monolithic upper/free float forend?  That should eliminate any flex.

With a ff tube and that much flex I would look at the receiver/barrel interface.

Did you hone the reciver and torque the barrel correctly? If you didnt, I would do that first, then look at alternatives.

If you want to take most of the flex out, get a steel upper reciever. I dont think thats needed since other HP guys dont use them that often.

I think after listening to all the folks that have offered advice & suggestions, that I
will purchase a VLTOR upper receiver that advertizes greater stiffness and then
learn to deal with whatever flex remains, if any.
I also intend to carefully mate the barrel to the receiver & torque it with great care.
Many thanks to the many that gave good advice & ideas.

Roger

Report your findings. And post pics.



 

i have a hard time believing your short thick upper is the problem and not your long thin handguard

apierce918;
That's what I thought too.  However empirical evidence (the target never lies) shows that flexing
was occurring.  When I thought about it, it's logical that a fulcrum point is where the support hand is on the
handguard, plus the support of the right hand while the sling is pulling down and back.  Remember that in
prone or less so in sitting, the rifle is being pulled down and back by the sling and held up by both right and
left hands
Consider that the (free floated) hand guard has the sling mount about 11" in front of the barrel nut.
My left hand hold in  prone is about 6" behind that and you have a lot of stress on the upper.
Couple that with the use of an inexpensive upper, (note to self, cheap parts are NEVER really cheap
on a rifle.) plus my use of a tight sling and I got flex.

Many thanks for all the thoughts and advice.

Roger

I see, I missed your post where you are using an optic, in which case handguard flex wouldnt matter