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Old October 13, 2013, 06:13 PM   #1
603Country
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Horizontal dispersion in my groups today

I was getting a bit low on ammo for the 223, so yesterday and today I did some reloading. And while I was at it, I wanted to try to find a good load with a new powder (BL-C(2)). Back when powder was tough to find, I managed to get several pounds of the BL-C(2) and figured that it should be fine for the 223. I've been using H-335 for a couple of years, but never hurts to try a new powder. The bullet in use is the Nosler 40 gr BT, with Lapua cases and CCI 400 primers. I started low on the charge and worked my way up and started seeing signs of success just under 30 gr of powder. The wife was after me to quit shooting and help her, so the last group I shot, with the load that has been looking the best, gave me a group with practically zero vertical dispersion, but about 3/4 inch of horizontal dispersion. I'd have shot a few more rounds, but I was under pressure from the wife so I quit. It'll be a week or more before I can get behind the gun again, and I know this is going to be on my mind.

So here's the question, and I'd prefer a response from the serious guys. Is horizontal dispersion more an issue of the shooter than it is the rifle and load combination? I'm thinking that the horizontal dispersion is mostly operator error.
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Old October 14, 2013, 09:21 AM   #2
Bart B.
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Chreighton Audette did some tests years ago with ammo whose case heads were out of square.

When the head's high point was indexed at the top or bottom of the chamber aligning with the closed bolt lugs, when fired at his 300 yard target, there was a slight amount of vertical stringing. With square case heads, shot groups were somewhat smaller in the vertical axis and about the same as horizontal ones.

With the head's high point indexed to the left or right, vertical stringing fairly well disappeared and group sizes in the vertical plane were the same as square case heads produced. But the horizintal shot stringing was 2 to 3 times as much as the vertical stringing with the high point at the top or bottom.

His reasoning was, the locking lugs supported bolt setback and minimized its effect on barrel whipping in the vertical axis with the head's high point indexed on them. 90 degrees away and indexed to the left and right side, there was no bolt lug there to resist movement so the barrel whip horizontally was a lot larger spreading shots out in that axis.

Makes sense to me, anyway. Especially after the US Army Reserve Rifle Team Captain told me they tried reloading 7.62 match case fired in their M14NM rifles, but accuracy was not nearly as good as what new cases produced. After measuring some fired cases for square heads, they all were a lot more unsquare than new cases which were pretty good. As none of their rifle's bolts had their faces squared up with the chamber axis, the out of square issue of both case heads and the bolt faces compounded the problem.
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Old October 14, 2013, 10:10 AM   #3
603Country
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Thanks Bart. That 223 is the only rifle I have that hasn't had the action squared up (not counting Dad's rifle and the grandson's rifle). I think I might as well have this one squared up also. It shoots pretty good, but just not quite good enough to suit me. Maybe I've gotten too picky about accuracy these days, but I won't say that's bad. I will say that it's expensive.

I'll try that BL-C(2) load a couple more times when I get the chance, but I'll still take the rifle to the doctor. That new Tikka T-3 with the Brux barrel has spoiled me on accuracy. I don't think I'll be happy till the 223 shoots that well also.
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Old October 14, 2013, 10:31 AM   #4
buck460XVR
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Without a chance to shoot the same ammo again in a controlled environment, one could only guess. My guess would be if you were shooting good groups up to the point where your wife was buggin' you to go home, the lack of focus and rushing your shots may be the determining factor. Could be in the rush to get the last of the shots off, your barrel warmed up more than usual. Maybe that particular load produced harmonics in the barrel leading to decreased accuracy. Odds are tho, it is as you suspect.....operator error.
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Old October 14, 2013, 11:00 AM   #5
Bart B.
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buck460XVR, I don't think barrel harmonics change with loads. The barreled action is the same piece of metal for every round fired. It vibrates at the same frequency for each load; harmonics are just higher multiples of that frequency and they stay the same, too. Heavier loads make it vibrate at slightly greater amplitudes but still at the same frequency as lighter loads.

Most of the barrel vibration's in the vertical plane 'cause the recoil axis is above the butt plate center; very little horizontally. Except when the way the receiver's fit to the stock doen't hold it repeatably side to side.

Last edited by Bart B.; October 14, 2013 at 11:50 AM.
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Old October 14, 2013, 09:42 PM   #6
Brian Pfleuger
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My .204 almost always groups horizontal. I've never known why. Even at 400 yards, my groups are like 1/2" high (at most) and 3" wide. Could be random chance as I typically shoot 3 shot groups.
However, when you've shot 10 3 shot groups and they're all like that, you've got a 30 shot group that's still like that.

I've read that it could be slightly incorrect parallax adjustment and/or failure to exactly duplicate stock pressure/holding the gun.

I suspect that it's a combination of mechanical error on my part and my failure to adequately read the wind between shots.
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Old October 15, 2013, 08:00 AM   #7
Bart B.
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One other thing about horizontal shot stringing; trigger finger placement on the trigger. If the finger's too far inside the guard on the trigger, that'll pull the rifle towards the shooter when the firing pin falls. If not enough, it'll push the rifle away. Ideally, the trigger finger should pull straight back on the trigger.

Good way to see which way the rifle jumps is go in position on the bench then watch the reticule jump as you dry fire on an empty chamber. If it jumps to the side at all, then change the point on your trigger finger tip that's on the trigger.

The heavier the trigger pull is, the more the rifle will jump to the side if the finger's not pulling straight back on it.
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Old October 15, 2013, 02:39 PM   #8
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Brian,

Assuming you have no stock contact, check bedding for bearing rearward harder on one side than the other (i.e., uneven recoil lug contact). Also take a felt marker (hi-spot blue is even more sensitive) and verify that the lugs are making even contact and don't need lapping. It sounds odd for lugs that engage vertically, but if they engage with one vertical (when closed) edge in contact and not the other, or if it isn't symmetrical contact for both lugs, this can introduce a twisting moment.
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Old October 15, 2013, 05:46 PM   #9
Brian Pfleuger
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Hm... interesting.

Some time ago, you and I had a discussion about something to do with my .204. I no longer recall the subject but I do recall measuring the recoil lugs and they ARE ridiculously uneven. Seems like I recall 0.016 but I wouldn't swear to it. I'll have to measure them again. I never made the connection.
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