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9/17/2020 5:59:48 PM
Posted: 7/4/2012 1:46:38 PM EDT
I was shooting with an old Marine buddy last week and testing his new AR build. All quality components. Mostly DD and some BCM I believe. Free float 18 inch barrel. Leupold 2-8 scope resting in a LaRue mount. Groups at 100 were 4-6 inches with a variety of different ammo. He remounted the scope and closed the gap to 36 yards on an old BZO target and the groups are keyhole and tight cloverleafs. Back to 100, again 4+ inches with quality ammo. I cant figure out what the hell would make a group open up that much by adding 65-ish yards. Thoughts?
Link Posted: 7/4/2012 1:50:34 PM EDT
too many variables.

different shooter?

check the bore?

check the crown?

you said it was keyholing but then you said it was making tight groups?  


Link Posted: 7/4/2012 2:00:16 PM EDT
Originally Posted By durabo:
too many variables.

different shooter?

check the bore?

check the crown?

you said it was keyholing but then you said it was making tight groups?  




By "keyhole" i meant successive shots in one hole @ 36 yards.

2 shooters had the same results.

Crown looked good to my untrained eye. No obvious damage.

I suppose the bore could have copper fouling thats not visible without a borescope.
Link Posted: 7/4/2012 2:04:40 PM EDT
[Last Edit: 7/4/2012 2:08:41 PM EDT by bigedp51]


This is a keyhole from an unstable bullet

Link Posted: 7/4/2012 2:05:45 PM EDT
What twist and what weight ammo?  40gr out of a 1/7 or 75gr out of a 1/9 could do it.
Link Posted: 7/4/2012 2:15:27 PM EDT


Agreed. But impact holes are nice and round. Wouldn't that rule out a over/under stabilized projectile?
Link Posted: 7/4/2012 2:17:16 PM EDT
We used to just call those "tumblers" when working the pits back in the day.
Link Posted: 7/4/2012 4:24:40 PM EDT
Originally Posted By mdarby:


Agreed. But impact holes are nice and round. Wouldn't that rule out a over/under stabilized projectile?


You are talking about a one hole group, when the hole gets small enough it is called a bug hole, rifles that shoot bug hole groups are expensive and used in competitive shooting. A key hole or holes are shot by worn out barrels or rifles with a damaged muzzle or crown.

Bug hole rifle.



Bug hole groups



Most of us are really lucky if we can shoot a bung hole group. (or even hit the barrel)



Link Posted: 7/4/2012 4:38:45 PM EDT
calling it a keyhole is going to confuse everyone and give you the wrong advise

I red loctite my barrel receivers to the upper though :noes:
Link Posted: 7/4/2012 5:30:07 PM EDT
Who built the rifle and most importantly who installed the barrel? If that was done improperly then nothing else matters.
Link Posted: 7/4/2012 6:57:34 PM EDT
It could be the scope getting screwy by adjusting the power range.   Maybe it doesn't hold zero at the 8 power but holds fine on 2.   That is IF you changed it for the different ranges.
Link Posted: 7/4/2012 6:59:17 PM EDT
Originally Posted By ErikS:
What twist and what weight ammo?  40gr out of a 1/7 or 75gr out of a 1/9 could do it.


OP, still need more data points.  Bullet grn.  Barrel twist.  Ammo maker.
Link Posted: 7/4/2012 7:09:48 PM EDT
[Last Edit: 7/4/2012 7:14:23 PM EDT by Invalid]
FROM  THE  TOP:
His use of the word "keyhole" means that several rounds made one hole that was shaped like a keyhole.  This we call a "group".
OP specified SEVERAL different types of premium ammo.  PLEASE read before you impart your wisdom.
I muse that improper barrel torque is as likely as anything.
It could be good to choose a load that works well enough with all barrel twists, like a premium 55 gr., and use that as your base line.
Link Posted: 7/4/2012 7:55:28 PM EDT
Had the same thing happen to me and I feel that I'm a decent shot, not the best, but decent enough.

When my groups opened up like you're talking about, it was due to a loose barrel nut. I about lost my mind that day when I was tryimg to zero my optic. I would get really tight groups at 25yds and then be all over the place out at 100yds.  Definitely give the upper a really good detailed inspection and hopefully things will work itself out.
Link Posted: 7/4/2012 9:29:43 PM EDT
Could be a something relate to scope parallax if everything on the rifle is mechanically correct. Not sure how experienced the shooters are so, if nothing looks wrong with the tool, it could just be the user. Scope parallax is the kind of thing people often don't realize is happening and can result in things like this.
Link Posted: 7/6/2012 5:32:33 PM EDT
Guys, I appreciate all the knowledge. Please dont confuse my misuse of the term "keyhole" as me being a total noob :) I am however new to building uppers.

I have a "bughole" shooting bolt gun and I get pissed when I shoot more than 3/4 minute groups with her. And yes.... its been a very expensive venture ($3600 so far lol).

The barrel nut seems to be the most likely candidate as a problem for my buddies rig. He assembled the upper himself. I assumed he used the proper torque specs on the barrel nut but i cant be sure.  If memory serves, it should be in the 80 ft lb range? I'm sure one of you guys know off the top of your head.
Link Posted: 7/6/2012 5:41:21 PM EDT
Originally Posted By mdarby:
Guys, I appreciate all the knowledge. Please dont confuse my misuse of the term "keyhole" as me being a total noob :) I am however new to building uppers.

I have a "bughole" shooting bolt gun and I get pissed when I shoot more than 3/4 minute groups with her. And yes.... its been a very expensive venture ($3600 so far lol).

The barrel nut seems to be the most likely candidate as a problem for my buddies rig. He assembled the upper himself. I assumed he used the proper torque specs on the barrel nut but i cant be sure.  If memory serves, it should be in the 80 ft lb range? I'm sure one of you guys know off the top of your head.


I got couple of questions: got pics of the group, ammo info, barrel info, and what type of upper rig you have? 80ft-lbs is pretty execessive for the barrel torque.
Link Posted: 7/6/2012 7:04:19 PM EDT
80 ft./lbs.????   EEEK!!!!  In the instructions for my free float tube, Armalite says 34 ft./lbs.  In

the industry section, someone asked why some Armalite rifles are much tighter than that.  

The answer was that the nut was turned only far enough to align with the next hole for the

gas tube.  Armalite specifies that you use a hi-temp anti sieze compound, because high

torque stress on Aluminum to Aluminum will cause galling.  Some guys say to use blue

loctite.  Doh!!!!  Once the gas tube is installed, the nut is there to stay.
PLEASE don't be the next guy to post a thread about destroying an upper because you

applied too much barrel torque!
Link Posted: 7/6/2012 9:21:25 PM EDT
LaRue mounts are adjustable to the picatinny rail. I had a quality scope in a LaRue mount on one of my AR's built to MK12 Mod 1 specs. I was getting horrible accuracy with match ammo (2-3" groups at 100 yards). I adjusted the LaRue mount to grab my rails but more tighter and the groups tightened up to around .5" at 100 yards.
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