Advanced Marksmanship Horizontal dispersion in a group a sign of what?

[TimK]

I've made some extremely minor changes to my load, but got drastically different results in testing today at the range.

The group on the left is the original load. All recent groups with this load resemble this one, all in the 3/8" to 5/8" range.

The two groups on the right were the modified load (neck sizing vs. FL sizing). All three groups were shot in a 20 minute period. Wind was strong, but seemed consistent. I didn't bother to measure velocity because I was thinking about other things.

I was working on achieving a natural POA and getting the rifle to recoil straight back. I was also loading the bipod for the first time. I was doing these things the same way (I thought) while shooting all groups. It was my first time working very hard on either, so I may well suck at it.

If I estimate the wind at 20mph, then it should only account for 1" or so of the dispersion.

If the load itself were screwed up, I should have seen both vertical and horizontal dispersion, right?

If that's true, then it's either wind...or me. Correct? If it's not wind, then what the heck was I doing wrong wrong? It sure felt like everything was the same...

 

[Lindy]

Re: Horizontal dispersion in a group a sign of what?

When we see two distinct groups like that on the left, it's often the result of the shooter repositioning while shooting the group.

The horizontal dispersion on the right may be somewhat due to the wind - a 20 mph full-value wind is a lot. But it may also be the result of poor trigger technique and/or a sub-optimum NPA.

On the plus side, your breathing looks very consistent. The groups are very small vertically.

 

[TimK]

Re: Horizontal dispersion in a group a sign of what?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">When we see two distinct groups like that on the left, it's often the result of the shooter repositioning while shooting the group.

The horizontal dispersion on the right may be somewhat due to the wind - a 20 mph full-value wind is a lot. But it may also be the result of poor trigger technique and/or a sub-optimum NPA.

On the plus side, your breathing looks very consistent. The groups are very small vertically.

</div></div>

There is no doubt I was repositioning myself, however the two shots together on the left target were the first and last in the string, whatever that means.

I'm sure my NPA is sub-optimal. I'm trying to get straight behind the rifle and just can't get comfortable.

 

[Lindy]

Re: Horizontal dispersion in a group a sign of what?

One of the things you may have to do is to reposition your scope so that your eye relief is correct when you're straight behind the rifle. That's especially likely if you used to shoot with your body at a significant angle in relation to the bore.

However, being straight behind the rifle will let you shoot better. It isn't necessarily intended to be comfortable.

 

[ChadTRG42]

Re: Horizontal dispersion in a group a sign of what?

+1 on what Lindy is saying about shooting position. That can make a difference in your consistancy. Always get behind the rifle the same way every time, and either load the bipod or not.

Before I read your post, I saw the pictures. I immediatly thought that he changed something in the load. Then I read you went from neck to FL sized ammo. This can make a difference. It can change up your pressures, which will effect the load and accuracy. It can also change up the neck tension, if you used a standard FL die vs. your neck bushing die. My personal load will vary by .2 grains in powder charge from a FL sized (with a neck bushing) and a neck sized only (same neck bushing). So, to make the loads shoot the same, I need .2 grains more powder in one of the loads (I can't remember off the top of my head which one). If you change anything in the load or brass prep process, you can effect how well your rifle likes/dislikes the ammo.

Also, when developing a load, you will often see groups string up and down and then progress to side to side (or vise versa). The sweet spot load is normally inbetween these powder loads. If you chrono the loads at the same time, you will often see a decrease in your ES, which again show a sweet spot. One of these days, I want to develop a load without shooting groups, and just use the chrono numbers as my gauge. I'm curious to see what the end result would be.

 

[TimK]

Re: Horizontal dispersion in a group a sign of what?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ChadTRG42</div><div class="ubbcode-body">
Also, when developing a load, you will often see groups string up and down and then progress to side to side (or vise versa). </div></div>

Crap, that's just what I didn't want to hear. I was hoping that side to side stringing meant it had to be an outside factor, not the load.

 

[sinister]

Re: Horizontal dispersion in a group a sign of what?

It appears your load is very, very good as the vertical dispersion is small.

The horizontal dispersion, shooting in 20+ knot wind, is all on you as the human component. Even your small 1/2" group is horizontally spread.

When shooting formal long range highpower team matches at 600 yards and farther the coach sits behind and between two shooters at a shooting point ("A Shooter" on the right, "B Shooter" on the left). Shooters are responsible for their elevation and the coach calls wind based on what he sees in his coach's scope -- each shot score horizontally is the coach's responsibility.

The old saying, "If you want to know how to shoot in wind, shoot in wind" certainly holds for New Mexico and West Texas.

Shoot your load when the wind is down -- I think you'll be pleased.

 

[TimK]

Re: Horizontal dispersion in a group a sign of what?

It was wind. I got out this morning and shot a very fast 5 shot group before the wind started to blow. I got four shots in dead calm, and the last in 7mph. Including the cold bore shot, my group measured 3/4". Not what I'm used to (3/8" - 1/2") but plenty good enough considering I shot it in the cold, on an empty stomach, after a heavy weight workout, and in a huge hurry.

Problem solved. Thanks for all the help.

 

[marduk185]

Re: Horizontal dispersion in a group a sign of what?

dont know what it means man but i do the same dang thing off a bipod, but get a rounder group from offhand or sling only sitting. id guess thats saying position related.

 

[TimK]

Re: Horizontal dispersion in a group a sign of what?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: marduk185</div><div class="ubbcode-body">dont know what it means man but i do the same dang thing off a bipod, but get a rounder group from offhand or sling only sitting. id guess thats saying position related. </div></div>

Do you load the bipod? I was trying that for the first time when I had my problem, and was wondering if that was a contributor. I'll do some more testing on that hypothesis when I have more than 5 minutes to shoot.

 

[Sterling Shooter]

Re: Horizontal dispersion in a group a sign of what?

Horizontal groups are the result of a multitude of problems, but, when wind, sight alignment, and trigger control are not questionable, NPA not being adjusted is a likely source of error.

Remember, recoil creates angularity from the line of bore at rest and the line of bore at bullet exit. This angularity must be consistent, which is achieved though consistent shooter control/contact of the rifle. Any change in the shooter's position from shot to shot, such as the elbow of the firing hand, perhaps a half inch off a previous placement, will bring about enough divergence in angularity to misplace the bullet. Of course, the effect of angularity increases with distance and is not always horizontal.

A relatively heavy rifle and/or light recoiling cartridge can reduce the effect; however, even in small-bore shooting adjusting NPA to perfection is still paramount to the best results. If there's time, always perfect the position for every shot, as even a minuscule amount of change in contact with the rifle is certain to misplace the bullet; and, without any regard at all to a consistent position, it's like every-time you pull the trigger you're establishing a new zero.

 

[jrob300]

Re: Horizontal dispersion in a group a sign of what?

It would appear you already have your answer, but mysterious horizontal spread was cured in my case by moving my right hand thumb from the "wrapped" position to resting on the tang, and making sure that my trigger finger was bent at 90* and pulling *straight* back. Once I made these two very small adjustments, my horizontal all but disappeared.

John

 

[Greg Langelius *]

Re: Horizontal dispersion in a group a sign of what?

Wind it is.

As initially presented, the problem would have appeared to be load-related. This is still conceivable for horizontal stringing, as barrel harmonics can result in oscillations in both vertical and horizontal planes. But that's apparently not at work here.

Another cause could be lateral loading of the trigger during the trigger pull, possibly combined with trigger finger pressure on the stock, which is referred to as 'dragging wood'.

The other suggestions above are all valid.

Finally, when you're concentrating on perfecting specifics, limit those specifics to one. If you do more than one and it works; then how would you know which one was making the improvement, or had been the problem?

The process of developing technique is to work on one specific at a time, honing it until it becomes effortless. Then move on to another, going about it the same way. Obviously, there will be holes in the technique until the process is fully addressed. We then repeat the cycle, working out each specific until we're comfortable with our ability, and then moving on to the next.

What we're doing is imposing one specific onto the foreground against a background of the other technical aspects. As all improve, each improves, and the obverse as well. It is by repetition of the whole cycle that the technical aspects become properly rounded.

But the <span style="text-decoration: line-through">only</span> best way this can work is by narrowing the concentration to a single aspect at any time of training.

Yes, we can chew gum and walk simultaneously, but before we got good at either, we worked on them one at a time.

Greg

 

[TimK]

Re: Horizontal dispersion in a group a sign of what?

Drat. I got out yesterday to confirm that my problem was wind...and it wasn't. I can't explain the fairly round (but large) group I shot the other day, but 5 out of 6 groups fired yesterday showed horizontal stringing.

I've loaded up some new ammo working up from 0.2g under the current charge up to 1.0g over in 0.2g increments. I've also loaded up 5 rounds using the FL sizing die at the current charge as that was the silver bullet load before.

Will report back as soon as the wind dies down here and I can try the new loads.