Do Test Groups' MOA Size Change With Range?

[Bart B.]

JohnKSa, nice explanation in your post #48.

[stagpanther]

Thanks guys.

[Roadkill2228]

As far as wind affecting things more if encountered in the first third than the last, yes the bullet spends more time getting pushed around in the last but a plain English way of saying this is that if it gets off to a bad start it affects the whole trip.

Even in the absence of wind groups often open up more rapidly than what a purely linear relationship between group size and distance would suggest (meaning lots of guns and loads can do half inch groups at 100 that wouldn’t even come close to doing 5 inches at 1000). One thing that really contributes to this might be imperfections in jacket thickness and bullet concentricity. At close range these don’t matter all that much (visibly deformed soft points shoot lights out to 200 - 300 yards in my 243) but over distance and thousands of rotations it shows. Think of a washing machine that for whatever reason ends up loaded heavier on one side. At first you can’t tell, but as the cycle continues and it spins more and more it gets shaky and clunky and loud and sometimes even moves around where it isn’t intended to be. Laundry ballistics. This lines up with real life observations a heck of a lot more than the theory about spiral shaped trajectories and what is statistically observed on targets with many shots fired.

[Bart B.]

Originally Posted by Roadkill2228

Think of a washing machine that for whatever reason ends up loaded heavier on one side. At first you can’t tell, but as the cycle continues and it spins more and more it gets shaky and clunky and loud and sometimes even moves around where it isn’t intended to be. Laundry ballistics.

The laundry center of mass moves around in its shape.

The bullet center of mass is fixed in place.

[Roadkill2228]

I must be missing something then. Wouldn’t it be the case that if
The bullet is not perfectly concentric/uniform then its center of mass wouldn’t be perfectly, well, centred?

[Bart B.]

Originally Posted by Roadkill2228

I must be missing something then. Wouldn’t it be the case that if The bullet is not perfectly concentric/uniform then its center of mass wouldn’t be perfectly, well, centred?

Yes. Very few of a given lot are perfect in that regard. Which is why 100 yard benchrest 5-shot group records are well under 1/10th inch but several groups average for aggregate records is much larger with the biggest group near 3 times larger. All the hundreds of other groups average bigger.

Some have spun bullets several thousand rpm checking balance. Perfect ones shoot most accurate.

[Jim Watson]

I used to read about the Vern Juenke Machine that provided a worthwhile sort on bullet uniformity. His son plans to make more.

[Bart B.]

Have a collet made to hold your bullet, then put it in a Dremel Moto Tool that runs at least 30,000 rpm

Connect an ampmeter between the Moto tool and power outlet.

Put a bullet in the upward pointed collet, turn the tool on.

Bullets out of balance will draw more current. Those way out of balance may fly out of the collet. Best balanced bullets draw least current, they are most accurate.

[cw308]

Bart B to your question . When I was looking for that pet load I only shoot at 100 yards . Then I found what I was looking for , I fired 3 shots dead center one hole , didn't have the balls to shoot the forth didn't want to spoil the look , kept the target . The load I'm using is the same to this day , never shot that tight again , now I only shoot 200 yards , for me shooting even 3 shots in one hole like that is going some . I'm still trying . In a 10 shot group some would be close but shooting one after another in the same hole , maybe if the rifle was bolted down , perfect conditions and without a human in the mix. Most of the other stuff , you guys are playing in deep water , way over my head . I'm an old trigger guy .

[RKG]

The analytical answer to the OP's query is this: group size factors that operate linearly should not cause an MOA delta based on range, while those that operate non-linearly will have such an effect.

This answer, though, isn't very helpful unless one knows what the group size factors are, knows which ones operate linearly, and knows what the coefficient of contribution of the various factors are.

Now, to revert to the real world. Wind can affect group size if it varies from shot to shot, or if it varies at different distances from the point of shot to the point of target impact. Or, as in the real world, both.

Comparing 100 yards and, say, 500 yards, wind variability is almost certainly a non-linearly variable. Even on a calm day.

But let's go one step further. Let us assume hypothetical conditions in which the direction and magnitude is both known and perfectly constant over the full 500 yards. Will the wind's contribution not be linear here? No. The influence of our hypothetically perfectly constant wind over any range is a function of bullet time of flight. Time of flight is not linearly related to range. As the bullet travels downrange, it is continuously shedding velocity, on account of drag.

So the time it takes the bullet to go 500 yards will be greater than exactly 5 times the time it takes the bullet to go the first 100 yards.

Did someone mention over thinking?

[Bart B.]

Here is some under thinking.

One non-linear variable is the vertical spread at different ranges caused by muzzle velocity spread. A 308 Win bullet drops about 0.2 inch (0.2 MOA) more at 100 yards for each 50 fps less muzzle velocity. At 1000 yards, it's about 20 inches (2.0 MOA) more. Drop in each 100 yards of range is directly portional to time of flight across each.

Whatever that is for the ammo can be changed by the LOF angle above the LOS for each shot as the bullet leaves. The muzzle axis angle vibrates mostly vertical. All bullets don't leave at the same angle to the line of sight.

[RKG]

And, just to complicate things further, there are factors not related to physics. The psychological factors.

I have an M14 with which I routinely shoot smaller MOAs at 300 or 500 yards than I do at 100 yards. Though I can't prove this, I am pretty sure what happens is that 100 yards seem so close that I rush or at least don't take as much care in sending my shots. On the other hand, longer ranges intimidate me into bearing down and being a better shooter.

[Bart B.]

One of the military teams observing the ideosynchrocies of M14NM rifles learned M118 match ammo MOA accuracy at 300 yards was not quite as good as 600 yard results. Their conclusion reached for cause was the muzzle axis angle at bullet exit compensated for velocity spread better for drop at 600 than at 300.

Such phenomena has been observed for over a century. Adjustable weights near the muzzle nowadays tune the bore axis vibration frequency to best compensate for velocity spread. Slower bullets leave at slightly higher angles than faster ones. Both center and rimfire rifles use these tuners.

[Unclenick]

I'll add, per Bart and John's descriptions on drift induced by wind deflection that what John said about that kind of drift applies to other kinds of drift equally. For example, suppose you shoot a bullet whose center of mass (CM) is off of the bore line a quarter thousandth of an inch due either to manufacturing imperfection or to the bullet being cocked a few moa inside the bore. Because this off-center CM orbits the bore axis as it goes down the tube, it picks up off-axis momentum that causes such bullets to open groups by flying away (drifting) from the bore line at a fairly constant velocity that isn't reduced appreciably by drag during the time of flight (TOF). This travel is at the angular its CM had going around the bore, and its direction is random if you can't control what side of the bore the CM favors when you chamber the cartridge. It's the same effect as spinning a weight on a string around your head and letting go of the string. The weight's momentum carries it in a direction tangent to wherever it was along the swing circle at the moment you let go of it. For the bullet, getting clear of the muzzle is being let go of. The angular velocity (the drift velocity) is the bullet rpm/60 (to get revolutions per second) times 2 times pi times the distance of the center of gravity was from the bore axis in inches. The result is in inches per second.

Here's an example I worked out in Excel. I took the time of flight for a bullet to travel each multiple of 100 yards distance from a ballistics program. The bullet was 175 grains and started out at 2700 fps. The only thing to note is that because the drift is constant and the time needed for the slowing bullet to go each successive 100 yards is longer and longer, the number of moa of drift increases for each successive 100 yards.

Note that drop per hundred yards changes partly for the same reason and partly because gravity is accelerating the rate of fall as time goes by. Also, note that moa in this chart is mathematical moa rather than the usual shooter's 1 moa/inch/hundred-yards approximation, but it just comes out about 4½% smaller, so no biggie. Multiply each entry by 1.0472 to get the approximation version.

[cw308]

There is alot to consider when working off charts . There are so many variables , I don't use a chronograph . When I find a load that shoots consistently tight groups at 100 yards in perfect condition I use that load from 1 to 2 hunderd yards , wind and temperature we can't control and human error is the only thing left . The rifle and round can only do so much , reading the wind is a talent . I keep it simple try not to over think , easy to blame the wind.

[old roper]

Nick, I have question. In 1000yd BR they shoot groups and score but on groups they can be less than 10".

https://internationalbenchrest.com/a...5%20062919.pdf

Those are match result. You have BR shooters with groups in the 4" range also. F-Class long range gets sighters same as BR.

It's nice to see things like this

http://bulletin.accurateshooter.com/...ass-nationals/

https://bergerbullets.com/national-records/nra-records/

[AzShooter]

I'm the fly in the ointment. When someone at the range tells me their gun shoots 1 MOA at such and such distance I tell them to prove it and bet a couple of bucks. I've never lost my bet. A lot goes into getting 1 moa. Not only the rifle but the skill of the shooter.

I've shot 1/2 inch groups at 1oo yards with my 45/70 but I would never say it was possible because my next group could be 3 inches just from trying too hard.

[Bart B.]

Originally Posted by AzShooter

I've shot 1/2 inch groups at 1oo yards with my 45/70 but I would never say it was possible because my next group could be 3 inches just from trying too hard.

You just told us it is possible. Some of us know it is possible once in a while.

[stagpanther]

Mine can shoot [] all day long.

[stagpanther]

So related to do this I'd like to ask you Bart--do you believe there is some kind of "auto-rotational stabilization" that happens with certain bullets that causes them to print tighter at 300 yds or so than at 100? I've heard top shooters weigh in on both sides of the argument and it's something I've always tried to understand. My personal opinion is that lacking control surfaces I don't see how that's possible.

[berettaprofessor]

Have a collet made to hold your bullet, then put it in a Dremel Moto Tool that runs at least 30,000 rpm

Connect an ampmeter between the Moto tool and power outlet.

Put a bullet in the upward pointed collet, turn the tool on.

Bullets out of balance will draw more current. Those way out of balance may fly out of the collet. Best balanced bullets draw least current, they are most accurate.

This is certainly something I've missed BartB, that sounds simple. Do you (or match shooters) often do this when loading for accuracy?

[Bart B.]

Originally Posted by stagpanther

So related to do this I'd like to ask you Bart--do you believe there is some kind of "auto-rotational stabilization" that happens with certain bullets that causes them to print tighter at 300 yds or so than at 100? I've heard top shooters weigh in on both sides of the argument and it's something I've always tried to understand. My personal opinion is that lacking control surfaces I don't see how that's possible.

Yes, that can happen. But not because of bullet spinning. Was first documented over a century ago by the British on their SMLE 303's accuracy at long range.

When all bullets leave on the bore axis upswing, slower ones depart on higher angles than faster ones because their barrel time is longer than faster ones. Their trajectories cross at some point down range. Slower bullets will be higher above the LOS than faster ones at about 5/8ths of the range where they cross. Then lower past the crossing point at target range

This is called positive compensation. Adjustable weights are sometimes put on barrels at their muzzle to adjust the bore axis vibration frequency tuning it for what is best for a given load at target range.

[Bart B.]

Originally Posted by berettaprofessor

This is certainly something I've missed BartB, that sounds simple. Do you (or match shooters) often do this when loading for accuracy?

I have not done that.

A few used the Juenke jacket thickness sensor that helped but it is no longer made.

The best commercial match bullets today shoot under 1/3 MOA at 100 yards, 2/3 at 1000. Group records are the smallest shot in matches

[stagpanther]

When all bullets leave on the bore axis upswing, slower ones depart on higher angles than faster ones because their barrel time is longer than faster ones. Their trajectories cross at some point down range. Slower bullets will be higher above the LOS than faster ones at about 5/8ths of the range where they cross. Then lower past the crossing point at target range

This is called positive compensation. Adjustable weights are sometimes put on barrels at their muzzle to adjust the bore axis vibration frequency tuning it for what is best for a given load at target range.

Thanks for enlightening me--so is this due in part to the velocity of the bullet, the gas impulse in the barrel, and the time the projectile exists in relation to that impulse??

[Bart B.]

It is best explained here:

https://www.varmintal.com/amode.htm

and check these tuner links on the left side of that page.

22LR Rifle & Tuner
Esten's Rifle & Tuner
Light Rifle & Tuner
Barrel Tuner Analysis