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Posted: 4/26/2014 6:28:53 PM EDT
I was in a competition today, and on the stages that ran 150yds up to 250yds, I noticed the following:
1) for the first six or seven shots, I was dead on
2) from shot eight and up, my POI was all over the map

Now, I know this is not a lot of information. The stages involved rapid shooting, so all shots on target were in quick succession...about a second between each.
I noticed the POI shift, time after time, even when I'd calm down, recenter and refresh myself.

Ammunition: 55gr Barnes HPBT with Benchmark handload 25gr
Upper Rec: Anchor Harvey and Rebel Arms 16" M4 government profile with short-length USGI-like carbine foregrip.
It was about 80F and pretty humid. Wind was not that much of a factor.

Ideas where to start going down the diagnosis trail?

Thanks.
Link Posted: 4/26/2014 6:46:36 PM EDT
How light is your bbl, and post groups from the range, hot & cold.
Link Posted: 4/26/2014 7:31:37 PM EDT
It's a light barrel. Not a heavy barrel by any means.
I wish I had the shots to post, but I do not.
After dead-on shots for the first seven rounds (seven seconds), the next group of four to six were about 1-3" high right and low left acc. to the spotter.
This feedback was consistent, stage after stage.

Again, thanks for the input based on little physical evidence!
Link Posted: 4/26/2014 7:39:30 PM EDT
Did you put the upper together yourself?
Link Posted: 4/26/2014 7:54:23 PM EDT
Originally Posted By gliderace:
I was in a competition today, and on the stages that ran 150yds up to 250yds, I noticed the following:
1) for the first six or seven shots, I was dead on
2) from shot eight and up, my POI was all over the map

Now, I know this is not a lot of information. The stages involved rapid shooting, so all shots on target were in quick succession...about a second between each.
I noticed the POI shift, time after time, even when I'd calm down, recenter and refresh myself.

Ammunition: 55gr Barnes HPBT with Benchmark handload 25gr
Upper Rec: Anchor Harvey and Rebel Arms 16" M4 government profile with short-length USGI-like carbine foregrip.
It was about 80F and pretty humid. Wind was not that much of a factor.

Ideas where to start going down the diagnosis trail?

Thanks.
View Quote


Free-floated handguard, or no?
Link Posted: 4/26/2014 7:58:50 PM EDT
It's an upper that I put together myself, yes.
The foregrip is not freefloating. It is a USGI (like) short-length kit.

I'm thinking that the barrel is getting hot and combined with the non-free-floating grip, I'm getting wobbles.
Link Posted: 4/27/2014 5:57:52 AM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By gliderace:
It's an upper that I put together myself, yes.
The foregrip is not freefloating. It is a USGI (like) short-length kit.

I'm thinking that the barrel is getting hot and combined with the non-free-floating grip, I'm getting wobbles.
View Quote


I've seen no better illustration of barrel harmonics than this video: DD Slo-Mo.

Observe carefully how the barrel "whips" during rapid fire; from the description of your course, your rifle may be displaying the same phenomenon, and a free-floated handguard may help. HTH.
Link Posted: 4/27/2014 6:29:25 AM EDT
Thank you, GSMullins. Good article; I was looking for the video, but could not find it, however.
Nonetheless, excellent read.

I think that you are right: the barrel is getting hot and not letting off heat quick enough. Thus, the wobbles.
I've been thinking of a free-floating barrel for quite a while, now. This may be the impetus to just go and do it!

Thanks!
Link Posted: 4/27/2014 7:27:18 AM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By gliderace:
Thank you, GSMullins. Good article; I was looking for the video, but could not find it, however.
Nonetheless, excellent read.

I think that you are right: the barrel is getting hot and not letting off heat quick enough. Thus, the wobbles.
I've been thinking of a free-floating barrel for quite a while, now. This may be the impetus to just go and do it!

Thanks!
View Quote


You're welcome, hope it helps.

It's not simply heat in the barrel causing the distortion, at least not at the rate at which you say you're shooting ("...about a second between each"), but the barrel responding to the pressure wave created at ignition. That 51K psi shock goes down the barrel at 18K feet per second, echoing back and forth from breech to muzzle until it reaches zero: when you execute a follow-up shot before that sine wave reaches zero, the new wave *can* interact so as to cause that barrel whip you see in the vid - note how for some shots, there's almost no movement, but the follow-up creates a big move. You can somewhat tune it out with heavy barrel contours (or continuous taper) or tailored handloads, but at some point the barrel's going to do what it's going to do. The free-float hand guard simply removes from the equation the effect of your hold, and what influence it has on the barrel, but it'll take more than that to damp the barrel movement. Just sayin'.
Link Posted: 4/27/2014 7:55:25 AM EDT
Not sure if this is the video he was talking about, but check out the lightweight barrel at about :25 - looks like a dang reed in the wind.

https://www.youtube.com/watch?v=0Xr_RG_lcNk

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