Odd Group Shape

[rg1911]

In testing loads (.223) in the new Krieger barrel for my AR-15, I ended up with a group shape I've never seen before and was wondering what would be the likely cause and if the load is worth pursuing.

At 100 yards, the 5 shots went into a 1-inch almost perfectly vertical string with each hole overlapping. In other words, if the vertical stringing had not happened, the 5 shots would have gone into one pretty small hole.

Conditions were gusting wind (although I tried to shoot during lulls), around 80 degrees F, and a pretty heavy mirage usually running right to left.

- Winchester cases and primers
- 55-gr Sierra Blitzking
- 23.6 grains AA2200 Data Powder (individually-weighed charges) (Since I have 30 pounds of the powder, it would be nice of it worked.)
- Average velocity: 3236 fps
- Extreme spread: 72 fps
- Standard Deviation: 28 fps

I'm a little surprised at the SD, since I did weigh each charge. All other loads using 60- and 55-grain bullets showed a "normal" group shape.

Thank you,
Richard

 

[29aholic]

Was the sun starting to set?

What is your front rest setup?

 

[Victor N TN]

A group like that is usually either a powder adjustment needed, or differences in sunlight. If the daylight didn't change intensity, change your powder just a bit up or down. Maybe 1 or 2 tenths. But it being perfectly vertical, there was no wind to speak of. Or you read the wind just right.

 

[gstaylorg]

If you can shoot multiple groups with the same shape, both repeatedly and consistently, then I'd be trying to figure out what was causing them to be that way. Otherwise, it's impossible to say anything meaningful regarding why a single 5-shot group had an unusual shape. A single group is statistically not nearly enough to say that wasn't a random pattern.

 

[Temp9]

If you can shoot multiple groups with the same shape, both repeatedly and consistently, then I'd be trying to figure out what was causing them to be that way. Otherwise, it's impossible to say anything meaningful regarding why a single 5-shot group had an unusual shape. A single group is statistically not nearly enough to say that wasn't a random pattern.

I agree. If you see consistent vertical stringing, we might have issues. Otherwise, I'm thinking it's just a coincidence.

 

[40xs]

Was the gun already hot from firing when you observed this string? The thing with a semi auto is the next round is chambered immediately after the last shot. If you take too much time to send the next round you could be "cooking" your powder, which will give you higher velocities.

 

[rg1911]

Thank you for all the tips. To answer a few questions:

1. That group was fired around 3:30 in the afternoon, so the sun was setting. While waiting for lulls in the wind, I estimate that it took approximately 10 minutes to fire the 5 rounds.

2. I was single-loading to avoid variations caused by changing pressure on the bolt, but each round could have sat in the chamber up to a minute. I was allowing at least a minute between shots to avoid overheating the barrel, and did not chamber a round until ready to shoot.

3. I agree that 1 instance is a statistical null and will see if it happens again with the same load. It was just so odd that I thought I'd check with the more-experienced members here.

4. The bullet used was actually the Hornady 55-gr V-Max; not the Sierra Blitzking.

5. The front rest is a very heavy one I bought years ago when I thought I'd get into benchrest shooting.

Cheers,
Richard

 

[airmonkey13]

Was the wind direction directly in front of you or left to right?


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