I Might Have a Problem Here

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I Might Have a Problem Here

Postby Synergy on Thu Apr 11, 2013 6:02 am

Was out at the range dialing in my new scope on my AR15. Had the gun on a front rest and sandbag in the rear and got is zero'ed and then all of a sudden it was shooting about 5-6" high at 50 yards. I finally figured out that when I had the hand guard sitting on the rest it shot really well (less then 1"groups at 50 yards with factory ammo) but when I inadvertently had the front of the gun sitting on just the barrel it was shooting about 5-6" high. I went back and forth several times and it was doing this consistently. Being new to AR's and rifles in general is this normal or do I have a problem with my gun? I suspect I need to re torque the barrel nut but if it was loose and moving around I would think my shots would be all over the place.

This is a home built AR, mega billet upper and lower, spikes barrel, midwest industries hand guard.
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Re: I Might Have a Problem Here

Postby Scratch on Thu Apr 11, 2013 6:18 am

Do you mean that when you had the barrel sitting on the rest, it shot badly? If so, then yes, that's what's supposed to happen. That's why free floating the barrel is so popular. It has to deal with barrel harmonics.
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Re: I Might Have a Problem Here

Postby engnerdan on Thu Apr 11, 2013 6:19 am

Synergy wrote:Was out at the range dialing in my new scope on my AR15. Had the gun on a front rest and sandbag in the rear and got is zero'ed and then all of a sudden it was shooting about 5-6" high at 50 yards. I finally figured out that when I had the hand guard sitting on the rest it shot really well (less then 1"groups at 50 yards with factory ammo) but when I inadvertently had the front of the gun sitting on just the barrel it was shooting about 5-6" high. I went back and forth several times and it was doing this consistently. Being new to AR's and rifles in general is this normal or do I have a problem with my gun? I suspect I need to re torque the barrel nut but if it was loose and moving around I would think my shots would be all over the place.

This is a home built AR, mega billet upper and lower, spikes barrel, midwest industries hand guard.


Its impossible to rule out something being loose but you NEVER want to support the rifle on the barrel because it will do exactly what you had happen. Plain and simple the barrel is flexing, which is throwing your shots off. The lighter the profile the more it will bend. It would only take about 2.3 degrees of bending to cause that much drift at 50 yards.

Sorry bad math.
Last edited by engnerdan on Fri Apr 12, 2013 11:39 am, edited 1 time in total.
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Re: I Might Have a Problem Here

Postby JFettig on Thu Apr 11, 2013 6:28 am

Check to see if your barrel nut is properly torqued. There is no way that barrel deflected that far by resting on it.
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Re: I Might Have a Problem Here

Postby Hmac on Thu Apr 11, 2013 6:33 am

It illustrates one reason why attaching the sling to a swivel on the FSB might not be a great idea. What kind of profile does this barrel have?

Image

However, a 5-6 inch change in POI seems kind of excessive to me and I'd loosen and re-torque the barrel nut if it were my rifle. Too loose or too tight can have negative effects on accuracy. I'd shoot for around 50 ft-lbs, but I confess I've assembled one or two rifles that took every bit of 80 ft-lbs to get the nut to index the gas tube properly.
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Re: I Might Have a Problem Here

Postby engnerdan on Thu Apr 11, 2013 6:34 am

JFettig wrote:Check to see if your barrel nut is properly torqued. There is no way that barrel deflected that far by resting on it.

I just did the math and JFettig is right, the tip of the barrel would have to move 0.639".

Sorry, bad math
Last edited by engnerdan on Fri Apr 12, 2013 11:40 am, edited 1 time in total.
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Re: I Might Have a Problem Here

Postby rugersol on Thu Apr 11, 2013 6:48 am

engnerdan wrote:
JFettig wrote:Check to see if your barrel nut is properly torqued. There is no way that barrel deflected that far by resting on it.


I just did the math and JFettig is right, the tip of the barrel would have to move 0.639".

I didn't do the math ... but that was my first thought, as well! ... that said, I've never checked the difference 'tween resting the bbl vs. the handguard, myself!
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Re: I Might Have a Problem Here

Postby AFTERMATH on Thu Apr 11, 2013 5:44 pm

I've only ever noticed a couple MOA poi shift when slinging up real tight on my m4gery... Sling on the sight base.
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Re: I Might Have a Problem Here

Postby Hmac on Thu Apr 11, 2013 6:08 pm

AFTERMATH wrote:I've only ever noticed a couple MOA poi shift when slinging up real tight on my m4gery... Sling on the sight base.


Yes, I agree. A good argument for attaching the sling to the handguard rather than a free-floated barrel.
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Re: I Might Have a Problem Here

Postby yuppiejr on Thu Apr 11, 2013 6:24 pm

When you installed the barrel/handguard nut how much torque did you use (ft-lbs)?
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Re: I Might Have a Problem Here

Postby farmerj on Fri Apr 12, 2013 12:38 am

rugersol wrote:
engnerdan wrote:
JFettig wrote:Check to see if your barrel nut is properly torqued. There is no way that barrel deflected that far by resting on it.


I just did the math and JFettig is right, the tip of the barrel would have to move 0.639".

I didn't do the math ... but that was my first thought, as well! ... that said, I've never checked the difference 'tween resting the bbl vs. the handguard, myself!



It makes a difference. That's why the SDM courses started to teach resting on the magazine and not on the barrel in the Army. Changes in the POI were significant.
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Re: I Might Have a Problem Here

Postby Synergy on Fri Apr 12, 2013 5:52 am

Thanks for all the replies

I had the guys at Atomic torque the barrel so I don't know. I'm going to pull it apart, clean it up and re torque it myself when I get a barrel nut wrench.

I do remember that on the midwest industries hand guard it came with its own barrel nut that has notches all the way around it that must be indexed correctly for the gas tube and so the hand guard lines up so hitting the exact torque value wasn't completely possible.
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Re: I Might Have a Problem Here

Postby rugersol on Fri Apr 12, 2013 6:21 am

farmerj wrote:Changes in the POI were significant.

12 moa?! :?
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Re: I Might Have a Problem Here

Postby rugersol on Fri Apr 12, 2013 6:29 am

Synergy wrote:I'm going to pull it apart, clean it up and re torque it myself when I get a barrel nut wrench.

I'd hold off ... a bit!

Most any bbl-nut has the same indexing/gas-tube issue! ... and I doubt Atomic Tactical F'd something like that up! At the very least, if ya think it ain't right, bring it back to them!

Even when it was first mentioned, I didn't think it was a bbl-nut issue! ... no reason, in particular ... jest don't seem right!

No offense, but 'fore ya go rebuilding the thing, I'd have someone else shoot it! ... ya never know!

If it weren't cycling, I'd suspect maybe the gas-block's loose?! ... even then, 12moa?!

Maybe re-check yer scope mount?! ... rings?! ... swap in a different scope?!

Like I said, I ain't tried it, myself ... but 12 moa at 50yd fer resting on the bbl?! :?
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Re: I Might Have a Problem Here

Postby Hmac on Fri Apr 12, 2013 7:28 am

Synergy wrote:Thanks for all the replies

I had the guys at Atomic torque the barrel so I don't know. I'm going to pull it apart, clean it up and re torque it myself when I get a barrel nut wrench.

I do remember that on the midwest industries hand guard it came with its own barrel nut that has notches all the way around it that must be indexed correctly for the gas tube and so the hand guard lines up so hitting the exact torque value wasn't completely possible.


There isn't really an exact torque value. Generally, torquing the barrel nut to anywhere between about 35 to 80 ft-lbs is acceptable. Depending on the barrel nut itself, many people will torque it to about 35 ft-lbs then go to the next notch to get the gas tube lined up, trying to stay under 80 ft-lbs. Lube the threads lightly with some moly grease. Tighten the nut down then loosen, repeat that a couple of times. If you go past the notch when tightening it down, don't back it off just a little, loosen it all the way the re-tighten.

However, if Atomic Tactical assembled it for you, I'd take it back to them, explain your problem and see if they can identify the issue and fix it for you.
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