Horizontal Deviation in Groups?

[3006loader]

When shooting longish range (100-250 yards), what causes horizontal deviation in groups? I understand that vertical deviation is caused by differences in velocity, which in turn is caused by charge amount differences, bullet seating depth etc, but what about horizontal? Is it just the wind?

[Jimro]

Wind, bullet imbalance, muzzle whip, how consistent the recoil is changing the angle of exit.

That's all I can think of off the top of my head.

Jimro

[cw308]

Could be wind, have you checked your scope rings,mount & action screws. Does your scope have a parallex adjustment? What is your horizontal group size ? Could be cheekweld, trigger press. Was it a shooter before this happened?

[Longshot4]

Give us all the info you can with what your dealing with.

[3006loader]

My groups as of yesterday shooting in the forest with mild to no wind are 2.5" at 200 yards. The group is about 2.5" tall and 2" wide. I am shooting a ruger american rifle with about 1500 rounds total through it. I am shooting from a steadyrest. I am guessing most of the horizontal spread is just from flinching/jerking the trigger. But all I wanted to know was what causes horizonal deviation and you guys answered that so thanks.

[603Country]

Unless the wind is strong, I usually assume horizontal spread is due mostly to me. But, recently I discovered that parallax on an old scope was causing some horizontal grouping problems. First time I had run into that problem.

[3006loader]

I'm also using a relatively cheap 4-12x scope so that also may be a factor.

[cw308]

3006loader
How often do you clean your rifle.? A good cleaning with a patch of Kroil oil followed by a patch of JB Compound, I'm a benchrest shooter, l clean my bore with KG - 1 for carbon followed by KG - 12 for copper. Four cleanings the fifth with Kroil & JB. Keeps bore clean & groups tight. You may be shooting with a fouled barrel.

[JeepHammer]

Something I probably bring up too much, but this will give you some idea of what's happening...

Aim dead center of a target at 25 yards and fire a 5 round group,
Do it again at 50 yards, and again at 75 yards, 100 yards, 125 yards, ect.

Verticle changes are range changes,
Horizontal changes, Especally if it starts left of center and moves right of center as you get past the 100 yard mark are more than likely your rifle on a cant,
Or your rifle barrel is cocked in the reciever, or the optics are cocked off centerline of the barrel bore.

Cant, or leaning the optics to one side or the other of the barrel center line can be addressed with a 'Spirit' level on the rail or optics tube.
It will teach you to strighten up the rifle as you shoot.
OR,
The optics tube is canted (rotated) in the rings.
Use a verticle 'Plum Bob' and high visibility string about 25 yards down range,
Line the BORE up on the string,
Use a 'Spirit Level' on the reciever to make sure the rifle itself is dead level...

Then see if your reticle verticle line is EXACTLY on the string or not.
With string dead center of bore, and reciever dead level,
The string should be exactly verticle with the verticle reticle line.

If not, time to correct optics tube (rotate) until the reticle line matches the plumb line.

If it still doesn't shoot mostly on ballistics tables,
Then consider the optics mount holes on the reciever aren't aligned with the bore,
Or the barrel isn't aligned with the reciever centerline/optics.
This is where a gunsmith can help strighten reciever threads, square the front of the reciever and make sure the optics mounting holes line up with the bore/reciever.

A string that continues to walk 'right' the more you shoot is usually a barrel cocked in the reciever,
The more you shoot, the more heat expansion and the further the barrel gets pushed 'right'...

If you let it get dead cool and it comes back 'Left' on its own, the walks 'right' as you fire it again, then its 100% a cocked barrel in the reciever.

[cw308]

JeepHammer
Very good advice, l had a scope with a canted reticle. Shooting at longer distances it did just that. Good Call.

[JeepHammer]

If it shoots 'Left' before 100 yards, then shoots 'Right' after 100 yards,
Its for sure the barrel cocked in the reciever.

If it angles, then it might be cant in the optics/rings or barrel cocked, either one.

Everyone seems to be stuck on using a level on the reciever, which is correct,
But they also use a level on a scope cap, which is dead wrong.
That cap/knob/turret isn't required to be square, it only turns a screw internally, so no reason for the factories to make them right angle true to the line of sight.

Without lasers, the ONLY way to ensure your tube lines up with the bore is to bore sight on a verticle string,
And gravity never lies!

The first spirit level on a rifle I ever saw was in the Marine Corps,
It was also the first time I ever saw a rifle shoot 50 yards to 1,000 yards right on the ballistics tables,
Check the ballistics table, dope the scope, point of aim is point of impact.
That's where I learned to true the reciever, check the barrel for true, and true the optics mounts...
Level the reciever, true the optics with a plumb bob line, and off to the races!

Suddenly the ballistics tables I'd been seeing for years made sense!
That was the first rifle I ever shot that would shoot smack on the tables.
If you missed, it was you and not the hardware or tables...

[cw308]

I set all my scopes with a plumb line and a level attached to every scope tube. Never had the opportunity in the Army to train with a scoped weapon. Starlight was the only scope. All our weapons were peep sights except the M79.

[3006loader]

JeepHammer, Good advice, but I think you misunderstood the question

[JeepHammer]

Might be, my brain breaks an axle and smacks the guardrail of confusion sometimes...

When someone talks about 'deviation', instead of 'stringing',
I think walking across, or in a diagonal, on the target with range increases.
The fastest way to confirm that is 25 yards starting, 25 yard increases out to about 300 yards.
Optics cocked on reciever, or barrel cocked in reciever will continue to walk with range.

'Stringing' to me means punching random holes more or less in a line, not consecutively walking in the same direction...

Under 300 yards, the elevation will move up/down, but you aren't ahooting far enough for 'Spin Drift' (hate that term) to effect the bullet too much.
Spin drift happens most when the bullet forward velocity slows significantly...
But rotation speed is still cooking right along.

Hope I didn't confuse things too much...

[firewrench044]

Other things that can do it are
very stiff trigger
jerking trigger
anticipating recoil
your head not in the exact same place behind scope each time
bad scope