Horizontal Stringing

[Swifty Morgan]

I posted the information in this post a couple of days ago, but it vanished, so I am trying again.

I shot my AR-15 at 50 yards, coming in at around 7/8" or about 1.5 MOA for 12 rounds. I moved to 100 rounds and shot from the ground, and I got about 2.5 MOA. I built a prone shooting platform and shot from 100 yards. I made a special effort to fix the scope's parallax. I will post the target. I shot 30 rounds at three bullseyes.

I started at the bottom left, and I didn't do too well. I found that the scope had come loose, so I fixed it. I also adjusted the windage. Then I shot the bullseye at the bottom right. Things were not a lot better.

This is how I remember things, anyway. It has been a couple of days.

Things started to come together when I shot the bullseye at top left. I got a very tight vertical dispersion with two flyers. Problem: horizontal stringing. I ended up with what looks like a snake. The holes are extremely close together, and they zigzag up and down.

If anyone can tell me what's happening here, I would appreciate it. It looks like this rifle will shoot sub-MOA, based on the vertical dispersion, but I'm doing something to make the bullets wander sideways. That's my guess, anyway.

The bipod (Magpul) allows the rifle to be leaned over, and it keeps leaning right as I shoot. I have to keep straightening it. I would rather have one that can be locked. I don't think this one will do that.

For all I know, something is going on with the gun. It's a CORE-15 M-Lok Scout with a Primary Arms 4-14x scope and a Midwest Industries quick detach mount. The rings were tightened with a torque wrench, and I did my best to attach the base according to the manual.

 

[stagpanther]

I have a core 15 upper--it's a chrome-lined barrel made by veterans who tend to lean more towards "battle ready" than "benchrest ninjas" so if it's giving you 1.5 MOA that's plenty fine IMO.

 

[Rimfire5]

Unless there is something wrong with the stock to barrel set up on your AR, I would suspect that it is the result of not maintaining a consistent set up from shot to shot.
Even with an semi-auto, even a .223 or 5.56x45, there is enough recoil to move the stock around on your shoulder and move your set up position. Larger calibers make the problem worse.

Horizontal stringing can occur, if you don't have the stock in the notch of your shoulder every time, and the stock is slightly toward the shoulder, the recoil will move the rifle to the right. The problem is exacerbated by AR stocks that are collapsible because they are rarely rigid. The standard A2 stock for an AR was designed for a person of normal size. About 1/2 of us are bigger than the median person the stock was designed for. When the recoil from a round is completed, you won't be in the position you were in before you fired, and you might not even have your eye in the optical center of the scope.
The next round can easily go further to the right because the stock is further to the right on your shoulder after the first recoil.

A bi-pod that seems to be tilted may further enhance the movement toward the tilt.

Vertical stringing might occur if your stock is a bit to short for you.
I experienced that with an A2 stock on my Les Bear Super Varmint .223. The first two rounds were in the same hole and the next three climbed vertically higher and higher.
Turns out that the short stock had me creeping up on the scope.
I solved the problem with a Magpul adjustable match stock. My 5-round group averages at 100 yards reduced from the high-0.4s to the mid-0.3s.
Top 25 loads average under 0.3 inches. Les Baer guarantees 0.5 inch groups with factory ammo. The two test targets, shot from a fixture, measured well under 0.2 inches. The difference in results was all the shooter, me.

And yes, the collapsible AR stocks that are so popular, are rarely long enough for a person with longer than normal arms to get sufficient stock length to avoid creeping up on the scope.

How to check?
Set yourself up so you think you are ready to shoot and then lean your head back slightly until you see the black ring around the scope image. If the black ring is not equally spaced around the rim of the image, then you are not in the optical plane of the scope and you need to move your set up so that the ring becomes concentric. Then move your head slightly forward until the ring just goes away, being careful to keep the ring concentric as it disappears.
You will then be in the optical center of the scope and will be at the correct eye relief so that you can repeat the next shot.

After you shoot a round, and think you are still in the same position, lean back again and see if you have to move back further to see the ring. If so, you inched forward and the next round will impact higher. If the ring isn't concentric any longer, you're being moved out of position by the recoil and the next shot will impact to the right or left, probably right.

I put a rolled up piece of tape on my cheek weld so I could experiment with the impact of the small increments that cause horizontal and vertical stringing.
It is amazing how little you have to move in your set up to get a 1/2 inch movement in POI at 100 yards.
About 1/8 inch of movement toward the scope can result in 1/4 inch rise in POI.
You can see the effect of not being in the groove of your shoulder by the amount of movement your point of aim is toward the right after your recoil. If you have to move the scope back onto your original point of aim, your shoulder is being pushed right by the recoil because you are not set up in the notch.
Even with high caliber rifles, having your stock in the right position on your shoulder will result in the scope coming back to the point of aim (maybe slightly higher) after recoil.

 

[HiBC]

Its hard to look at a 10 shot group and ask "What did I do to make that happen?"
Fire one shot groups. Analyze what happened in the moment, Call each shot.

You might need a good spotting scope setup

 

[Swifty Morgan]

Thanks for the help.

RE the rifle, ordinarily, I would think it was fine to shoot this well with this gun, but given the fact that my group was excellent vertically, it just seems natural to try to fix the horizontal issue.

 

[Bart B.]

Horizontal shot stringing is most often caused by something making the barrel point different lines of fire upon bullet exit from the barrel. Most often by the rifle not being held the same way for each shot.

Make sure you shoulder and hold the rifle exactly the same for each shot. Different elbow positions for every shot cause horizontal shot stringing. And pull the trigger straight back.

 

[COSteve]

Horizontal shot stringing is most often caused by something making the barrel point different lines of fire upon bullet exit from the barrel. Most often by the rifle not being held the same way for each shot.

Make sure you shoulder and hold the rifle exactly the same for each shot. Different elbow positions for every shot cause horizontal shot stringing. And pull the trigger straight back.

This +++

 

[Bart B.]

Remember that the barrel doesn't point to somewhere above the line of sight to the point of aim equal to bullet drop at target range plus sight height above the bore axis until the bullet leaves the barrel.

When the primer fires, the bore axis points X MOA lower below the LOS and X MOA left or right.

As your stuff's accuracy improves along with your marksmanship skills, you'll see the difference in zeros for benchrest and field positions.