Why do shot groups open up over range?

R

rainydayshooter

New member
While saving up and shopping to get my rifle re-barreled and bedded, I've been trying to mentally construct things.

Given the round is capable of the pursued range,
if a rifle holds a good group at 100 yards consistently, then the groups start to break up at a certain distance beyond the typical 10-15%/100 yards, what's to blame? and what if there is no certain yard, meaning hypothetically, if I can shoot sub-moa at 100 yards only and my group opens up 25% every 100 yards?

Dismissing abnormal weather conditions, I imagine this has to do with the ballistics, rifling, and stability of the bullet. So I would blame either or both the barrel and projectile. I imagine a poorly bedded action could be eliminated since this would result in fliers even on the hundred, but what about inconsistent charges? or inconsistent chamber pressure? Would these be hidden on the hundred and not show up until certain distances?

I'm wondering what causes the rounds accuracy to loosen up along their trajectory, and what it means if the round suddenly starts losing accuracy at a certain yard, or consistently loses accuracy past the 10 -15% pattern for every hundred?
 
Notice it begins once the bullet slows to sub-sonic. Also the more air it goes through, the more variations it goes through.
 
The answer is TIME.

There are lots of forces acting on a bullet to cause it to not hit in the exact same hole each time. The best you can do is to manage those forces (more exacting powder measurement, managing barrel oscillation, tighter tolerances for bullet weight,etc). However much you do to minimize them though, those forces are still present. Then, there are the forces that you can't minimize yourself - like wind.

The more time that bullet spends in flight, the longer those forces have to act on the bullet. Thus, a set of forces that would create a .5" spread at 100 yards would be able to create more than a 2" spread at 400 yds.
 
And Doyle finished it. Little things over time, become bigger things. Tiny differences in bullet weight, bullet shape, air speed, air density, temperature, powder weight, power distribution within the cartridge. The effects of a 2nd shot through a barrel that just had another bullet it in that left bits of residue, the 3rd with cumulative residue...... All that stuff matters exponentially more over time, and thus, distance. That effects groups. I did mention the drop to subsonic, but that really doesn't hurt groups as much as the accuracy itself. The only way that hurts groups is the different distances per shot that drop takes place, which is more of a factor of all those other things. Drift, you can correct for.
 
Back to the OP:

Are you meaning the physical group size gets measurably bigger with range (as anything going out at an initial angle would)...
Or that the angular dispersion itself gets bigger ?
 
thanks for the replies.
the time spent battling air makes sense to me.

I'm still trying to sort out the trajectory formulas used.
Not sure if any ballistic calcs use the parabolic trajectory formula.

The reason I ask this question, is because I have a rifle that is doing just this. It consistently holds sub-moa at 100 yards, but it falls apart somewhere along the way out to 1,000.

My initial fear in upgrading this rifle for roughly the same cost as a new one, is building on an action that doesn't have a clean lock. My scale does bounce .2 grains off when loading my rounds but I also wonder if there could be a mechanical imperfection.

Among component imperfections found, my cases are about 10 grains off from each other, but I do find the projectiles to be consistent with occasional change of .1 grains (according to my digital scale)

If it were inconsistent velocity, I would think that the lost velocity would make a larger deviation on the way down as gravity accelerates, meaning if I adjust the elevation creating an arch to achieve my target, then if the charge of the 2nd round were off by say 50 fps then I would have a larger percentage of error at 1,000 yards vs 200, but surprisingly this seems less significant than I suspected according to an online ballistic calculator's results. The difference between 200 yards and 1,000 is only 1% resulting in apx. 2" deviation at 1,000 yards. Now if I could plug that into a trajectory formula representing the arc for zero at a thousand what would it be???

I'd like to learn to use the equations at work behind the scenes but right now I'm thinking a better projectile and barrel with better rifling and velocity will be best.

Back to the OP:

Are you meaning the physical group size gets measurably bigger with range (as anything going out at an initial angle would)...
Or that the angular dispersion itself gets bigger ?
Click to expand...

Correct. The percentage of error increases with range. Well past the common 10-15% even on a phenomenal day.
 
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"...Correct."
Click to expand...
It was an either/or question. :rolleyes:
"Yes" is not an option. :D (unless you're a Democrat.)

Are you are saying that a 1" group at 100 becomes significantly larger than 5" at 500 ?
(Hint: That's a 1-or-zero question too. ;) :cool: )
 
Correct. It's wrong. It's all wrong.
500 doesn't group 5" more like 8" and 600 isn't 6" it's more like 15" if I recall. By the time I'm at the 1,000, I'm grouping 35".
 
As range increases the crosshairs on your scope cover more and more of the target making precise aiming more difficult Benchrest shooters are only concerned with group size and adjust their scopes so that the bullets are impacting several inches from the aim point. If your bullet holes are hitting where you are aiming they destroy the aiming point for following shots.

I find a square aiming point helps with smaller groups too. I can place the crosshairs on a corner and line up the vertical and horizontal crosshairs with the square for a more precise aiming point for each shot.

And as others have said the longer the bullet is in the air the more time wind and other factors have to mess things up. If your ammo's speed varies by 10-15 fps from shot to shot it might mean a difference of .1 MOA at 100 yards and most people don't even measure that precisely. At 500 yards you might see .5 MOA difference. And 10-15 fps difference is pretty good ammo.
 
Rounds leave your barrel at slightly different angles each shot. They also have different velocities.

At short range, angle of departure is the primary issue related to group size. A 1 MOA group at 100 yds means you and your barrel spit bullets out within about 1 MOA of eachother.

At 500 yds, wind effects and trajectory variation adds in. A 1 MOA group at 100 yds should be 1 MOA at 500 or 5", but 4" of wind drift and 3" of trajectory variation caused by muzzle velocity now have you shooting 10" groups.

At 1000 yds, more variables need added in too. That is why a 1 MOA group at 1000 yds is fine shooting and to get the same accolades at 100 yds, you need a .3 MOA group!
 
It's a .270 150 grain and I'm using loads between 2600 and 2750 fps.

I have a long thread about it on here, I've been working on for years.
I'm looking to get the barrel installed and action done by the end of this year.

Just trying to get a better understanding of the behavior as I wait.
 
According to JBM, that bullet goes subsonic at 900 yards when starting at 2750 fps so performance at 1000 would be mediocre.

Optimal performance (accuracy wise) would be out to 700 yards after which it starts going transonic.

Below those ranges that bullet is still affected by wind which could explain the larger groups you are seeing. A full value side wind of just 7 mph would push that bullet 0.6 inches off center at 100 yards, 5.6 inches at 300 and 26 inches at 600.

So unless you are shooting in perfectly stable conditions, the POI would be likely to move much more (percentage wise) the longer the distance.
 
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JBM? Thanks for sharing this.

I was using gundata.org and hornady and it was still showing supersonic.

This is interesting because velocity has been my theory for a couple other reasons. Someone on the range said the behavior was leading them to believe I was going subsonic, but the gundata hornady calculator states otherwise.

Certain things haven't been adding up, and I'm not even convinced my hotter loads are generating anywhere close to 2,750 fps mv.

Update:
That was the hornady ballistic calculator which included velocity and showed it being supersonic. I was also using the gundata.org calc for bullet drop comparisons.
 
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Sierra's software says that bullet's over 1200 fps at a thousand yards. Its exterior ballistics down range are:

A 100 fps spread in muzzle velocity with that bullet causes about a 2/10ths inch spread in drop at 100 yards. At 1000, it's about 3 feet. That's a cause of vertical shot stringing.

A 2 mph variable in cross wind makes it horizontally string about 1/6th inch at 100 yards. It'll string sideways almost 2 feet at a thousand yards in those conditions. Crosswinds and air densities are never constant in the bullet's trajectory unless you're shooting at 4 AM long before the sun heats up the atmosphere and it's dead calm; a good time to test stuff for long range accuracy.

At 500 yards, those condition's variables mean about 5 inches vertical spread and 4 inches horizontal spread.

What's your bullet's muzzle velocity spread and what's the wind doing in its trajectory?
 
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A 100 fps spread in muzzle velocity with that bullet causes about a 2/10ths inch spread in drop at 100 yards. At 1000, it's about 3 feet. That's a cause of vertical shot stringing.
Click to expand...

Thank you!
I'll have to purchase this. Looks like it's only $40 and you actually get a disc!

I don't know how accurate the manual is, because I had to adjust elevation far too much to reach target than the manual stated I needed. The recoil felt light too. I'll have to check barrel lengths used.

Last day out the flags were still with a little rustling around the drooping tips. Mirage wasn't bad. I don't think my loads were that off but 3' off is about what I was shooting but it varied 15" all around the target. I will log every shot next time to verify which direction most were falling off.
 
Note that the crosswind speed's not the same above the line of sight as in it. Wind blows faster the higher it is above ground. It varies as much as 50% from zero to a dozen feet above it depending on the terrain; flat ground has less difference than landscape obstructed by trees and hills.
 
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