GROUP SIZE AND RANGE

2.4K Views
Last Post 06 February 2016

CBA Member

joeb33050 posted this 16 January 2016

I expected group size to be proportional to range, at least to 200 yards or so. I was wrong.

Group sizes at 200 yards are greater than 2 times 100-yard group sizes. For the 506 CBA National Match records from 2000-2013, 5 shot groups at 200 yards are 2.236 times the size of 5 shot groups at 100 yards. The ratio for 10 shot groups is 2.385.

Group size MOA increases as range increases, at least in those CBA matches.

Taking that, I assumed that “conditions” caused the increase, and worked out an elaborate and riviting analysis of the effect of wind and shooter compensation for wind.

Then “husker” presents data at 50 and 100 meters for 10 shot group size shot in a tunnel.

This, husker's, data, if representative of a larger sample, means that my wind analysis is wrong. I'm wrong again.

(Assume constant bullet forward speed.) If group size, in MOA, increases with range, then either something extraneous must happen to the bullet, or a speed caused by an acceleration = vector must be imposed upon the bullet between muzzle and closest target.

Wind is an extraneous force that operates on the bullet between, say, 100 and 200 yards. A 1” 100 yard group would turn into a > 2” 200 yard group if the wind continued between 100 and 200 yards. If there were no wind between 100 and 200, and no other effect, the 1” would become 2".

A speed caused by an acceleration = a vector, is imposed on the bullet by the wind between 100 and 200 yards. If the bullet is moved 1” in 100 yards, that speed-call it a sideways movement for example, is 1” per hundred yards. That sideways speed will continue from 100 to 200 yards.

First, group size doubles as range doubles, a 1” group becomes 2". But, since the wind has put a sideways speed on the bullet of 1” per hundred yards, the group becomes 3" And, since the wind is still blowing on the bullet, another inch makes 4” groups. So, wind drift varies as the square of distance, and that's about what wind drift calculators show.

BUT

Group size increases with distance and NO WIND, as husker's data shows. Something's happening, and I don't know what it is.

We need either an extraneous force on the bullet from D1 to D2, or the bullet to be moved sideways, to accelerate.

Mann's explanation of x and y on page 256 doesn't show either.

I find the introduction of data to be most upsetting to my preconceived notions.

Attached Files

24 Comments

Order By: Standard | Newest | Votes

CBA Member

Scearcy posted this 16 January 2016

Joe, for years I have tracked all of the matches in Region 5. We hold 13-15 matches per year with rifles in every class including military. Shooters come and (sadly ) depart. Most shooters migrate from one rifle class to another over time. Conditions vary from match to match, etc. Your ratios for 5 and 10 shot groups are almost exactly the same as ours have been.

Attached Files

CBA Member

45 2.1 posted this 16 January 2016

You might consider that what you're trying to find will not show itself at 200 yards and less. I shoot HV in bolt and semi-auto besides slow heavy bullets in single shots at longer ranges. Seldom will bad loads or bullets really go bad, or show what you're asking for, under 300 yards in my experience. Of course, the loading regimen and slow twists used here won't really allow testing “out there". My main calibers of concern are the 308/7.62 and the 45-70. Both exhibit the “going to sleep” syndrome past 200 yards where the MOA group size gets smaller than those shot at 200 and under. I don't shoot conventional loads either, so cannot comment on those where one uses powders normal for the cartridge. Something you should consider............

Attached Files

CBA Member

RicinYakima posted this 16 January 2016

Hum, that will keep me thinking for the rest of the day, Joe.

Attached Files

CBA Member

joeb33050 posted this 16 January 2016

45 2.1 wrote: You might consider that what you're trying to find will not show itself at 200 yards and less. I shoot HV in bolt and semi-auto besides slow heavy bullets in single shots at longer ranges. Seldom will bad loads or bullets really go bad, or show what you're asking for, under 300 yards in my experience. Of course, the loading regimen and slow twists used here won't really allow testing “out there". My main calibers of concern are the 308/7.62 and the 45-70. Both exhibit the “going to sleep” syndrome past 200 yards where the MOA group size gets smaller than those shot at 200 and under. I don't shoot conventional loads either, so cannot comment on those where one uses powders normal for the cartridge. Something you should consider............Different story, not the one that's got me flummoxed. (Can we say flummoxed?) We've got data showing it DOES happen, unfortunately we've got data showing it happens with no wind. After 200 yards/going to sleep is another matter, UNLESS you can connect the two. Thanks; joe b.

Attached Files

CBA Member

joeb33050 posted this 16 January 2016

I've had the Mann picture in my mind for 40 years, from behind, the bullet comes out the muzzle a little off, then describes a helical path going down range. The base moves, from behind, in a circle. These two cause inaccuracy-then there's the wind and shooter and technique and... But, my picture/Franklin's picture, may need adjusting.

Attached Files

CBA Member

frnkeore posted this 16 January 2016

It's also perplexed me. I've always chocked it up to conditions. In my disipline, we don't shoot 100 very often but, 3/4” 100 yard groups can be producted pretty often where as a common 200 yard group will be around 2” for 5 shots.

2 years ago, last Oct, I shot a 5, five shot match @ 100 in Modesto. The winner shot a average of .388, setting a new record but, his 200 yard 10 shot group, the same day, was 1.5">. Using John's math above, his 200 yard 10 group, should have been under 1". Especially since his small group was in the .2's. Not a good comparison, I know but, I see this type of thing a lot in my matches.

Wind drift is more of a problem at our velocity's as is getting the actual “0” settled in, in score shooting.

Another factor in using shooting records for obtaining group sizes, is that when shooting group or score, you usually or at least, many times, hold off a little after firing your second shot, to compensate for what ever you think the next POI will be, based mostly on conditions. So, in records, especially, your getting a shooters skill involved.

No easy answers to this question and I believe they'll only come with good equipment and a tunnel :(

Also, I think 50 yard shooting dosen't compare very well with 100. RF maybe the exception to that but, in CF cast, I see a lot more variation from 50 to 100 groups. In the 80's and 90's, I used to do all my testing at 100 but, have always shot mostly at 200. But, it wound up biting and now I do all my serious testing 200. The reason for the 100 yard testing was two fold, first was that, that was what Mann used and second was that I could get more groups on a target at 100 and of course the walk :) Rambeling, I know.

Frank

Attached Files

CBA Member

muley posted this 16 January 2016

joe, I am no math man , but with the wind and all other external variables, would there be more cnsistancy if the bullet remained at the same speed from muzzle to 200 yards?

Attached Files

CBA Member

Ken Campbell Iowa posted this 16 January 2016

here is something that ( might ) explain some non-linear dispersion with respect to range.

it is dispersion with time.

because the little nudge the unbalanced bullet gets at muzzle exit .... is a vector of sideways distance in repect to time.

if the nudge is 1 inch per second ... and the first 100 yards takes one second then the additional deflection is 1 inch.

however the 2nd 100 yards takes the now slower bullet two seconds so the deflection is an additional 2 inches ... a total of 3 inches ... appearing to be non linear in respect to distance ... although linear in respect to time .

i think .

ken

Attached Files

CBA Member

RicinYakima posted this 17 January 2016

Ken, Is that the same principle of wind drift function? The more and faster the bullet slows, the more effect the wind has upon it? Ric

Attached Files

CBA Member

45 2.1 posted this 17 January 2016

joeb33050 wrote: 45 2.1 wrote: You might consider that what you're trying to find will not show itself at 200 yards and less. I shoot HV in bolt and semi-auto besides slow heavy bullets in single shots at longer ranges. Seldom will bad loads or bullets really go bad, or show what you're asking for, under 300 yards in my experience. Of course, the loading regimen and slow twists used here won't really allow testing “out there". My main calibers of concern are the 308/7.62 and the 45-70. Both exhibit the “going to sleep” syndrome past 200 yards where the MOA group size gets smaller than those shot at 200 and under. I don't shoot conventional loads either, so cannot comment on those where one uses powders normal for the cartridge. Something you should consider............Different story, not the one that's got me flummoxed. (Can we say flummoxed?) We've got data showing it DOES happen, unfortunately we've got data showing it happens with no wind. After 200 yards/going to sleep is another matter, UNLESS you can connect the two. Thanks; joe b. I see no tests past 200 yards in the benchrest discipline. I would note that most rifles have slower twists. I'm shooting box stock rifles with 10 or 11 twists in 308/7.62. As an example, I re-sighted my Ruger GSR after banging it and myself on some creek ice...twice. I re-sighted at 100 and got my normal 5/8” to 3/4” 5 shot group on paper, then shot my painted gong at 257 yards with a result of a 1-1/8” 5 shot group. Mind you it was about 30 degrees with a 20 mph wind. The load was a MP 308 Hunting Bullet (my design) going ~2,100 fps (any faster is much to destructive on hides). I think a marginally stabilized bullet is the problem you guys are having myself. You tell me why it's happening as what I related is normal results over the year for the rifle and load (and it does the same for my HV loading as well).

Attached Files

CBA President

John Alexander posted this 17 January 2016

Joe, I wouldn't jump to conclusions just yet. The tests at the Lapua lab were just on rimfire between 50 and 100. Maybe good cast bullets act differently and jacketed bullets even more differently. Ballistic experts must already know about this and what the parameters are that affect it. Because CBs don't seem to be too far from linear expansion between 100 and 200 and wind and mirage must be responsible for some (or all) of it. As I remember you were surprised at how little the group expansion was from 100 to 200 when you were working on it.

I doubt that anyone has done anything similar to the Lapua test for CB but I'm bet people working on military stuff have done the same. Do the bullets they test cause groups to expand linearly with distance. Maybe we can find out. I expect that wheel has already been invented.

John

Attached Files

CBA Member

Ken Campbell Iowa posted this 17 January 2016

ric :: kinda the same as wind drift, but the initial nudge momentum at the muzzle will remain pretty much constant ....whereas wind drift is much more complicated, especially in practice, where the wind really swirls more than blows in a predictable direction ....

ken

Attached Files

Standard Member

Hamish posted this 17 January 2016

I wonder, did any of the Houston Warehouse shooters ever mention the “sleep” phenomenon, or anything concerning the dispersion qustions under discussion?

Attached Files

CBA Member

joeb33050 posted this 17 January 2016

Ken Campbell Iowa wrote: here is something that ( might ) explain some non-linear dispersion with respect to range.

it is dispersion with time.

because the little nudge the unbalanced bullet gets at muzzle exit .... is a vector of sideways distance in repect to time.

if the nudge is 1 inch per second ... and the first 100 yards takes one second then the additional deflection is 1 inch.

i think .

kenken; I'd appreciate it if you would take a little time before replying, maybe a few false starts. Otherwise your rapid and correct response makes me look like a goon for not seeing this earlier. You got it, thanks. I'm going to sulk now, and be quite cranky. joe b.

Attached Files

CBA Member

joeb33050 posted this 17 January 2016

TheAmishGolfballShooter wrote: I wonder, did any of the Houston Warehouse shooters ever mention the “sleep” phenomenon, or anything concerning the dispersion qustions under discussion?Why would anyone shoot a warehouse? Oh, Houston. Got it.

Attached Files

CBA Member

joeb33050 posted this 17 January 2016

muley wrote: joe, I am no math man , but with the wind and all other external variables, would there be more cnsistancy if the bullet remained at the same speed from muzzle to 200 yards? Yes. Wind drift varies with the amount of speed LOST between muzzle and target. The John Alexander approach involves lower velocity, long bullets and fast twists to minimize speed lost.

Attached Files

CBA Member

Ken Campbell Iowa posted this 17 January 2016

houston warehouse adventure :: i have read a couple reports of that and don't recall mention of causes of dispersion from events after free launch of the bullet .

to perhaps cloud my above thought on linear dispersion per time ..... it would seem to me that perfect bullets would not receive the nudge at exit .... and the worse the cg/cf mismatch of the bullet the bigger the nudge ....heh, so with benchrest quality mj bullets there would be little nudge momentum, and so the exponential graph of dispersion with distance would not be seen .

if you find a linear with distance cone of dispersion, your bullets exit perfectly. so, like flashbulbs, you find out after shooting the quality of your bullet .

yep, this is boring ... as are most things that are true ... and so our attraction to cast bullets , .... maybe because the facts that we consider are seldom true ?? i have know many wimmin critters like that ... they usually deviate logarithmic ally ...

oh heck while i am at it ... if the nudge is centered axially, the nudge would just push the bullet sideways .... but if the void-error is at the base ( or nose ) , it would try to induce a wobble ... i think ... and so affect b.c. ..... therefore lower impact and higher dispersion from wind .

and ,,, in regard to 22 rimfire ...the real question is why do they shoot so good .... they fail in all the common sense requirements for accurate bullets .

ken

Attached Files

CBA President

John Alexander posted this 17 January 2016

      Joe wrote:

ken;

I'd appreciate it if you would take a little time before replying, maybe a few false starts. Otherwise your rapid and correct response makes me look like a goon for not seeing this earlier.You got it, thanks.I'm going to sulk now, and be quite cranky.joe b.

Joe

I'm impressed too, but don't give him too much credit or he will get uppity. So according to Campbell's Principle, a defect free bullet (no little nudge at the muzzle) should fly straight (unless there is something not yet considered in this discussion.) and thus produce linear expansion of groups in a tunnel. The bigger the defect the the more non-linear. But of course there ARE other things that impart a velocity vector to the bullet perpendicular to the line of flight, barrel vibrations being one. So just considering these two possibilities, a very very stiff barrel and perfect bullets would produce groups that expand linearly in a tunnel. We have mercifully avoided talking of spiral bullet paths, “going to sleep” and little cast bullet demons that push on bullets with their hind legs. Hmm, my copy of Vaughn's book was stolen and they are now $500 on Amazon. Can anybody recommend a similar book on ballistics? John

Attached Files

CBA President

John Alexander posted this 17 January 2016

Ken wrote:

There is no end to this guys good thinking. I told you he would get uppity.

John

Attached Files

CBA Member

joeb33050 posted this 17 January 2016

John Alexander wrote: Joe wrote: ken;

I'd appreciate it if you would take a little time before replying, maybe a few false starts. Otherwise your rapid and correct response makes me look like a goon for not seeing this earlier.You got it, thanks.I'm going to sulk now, and be quite cranky.joe b.

Joe

Vibration? Data? What are others? We have mercifully avoided talking of spiral bullet paths, “going to sleep” and little cast bullet demons that push on bullets with their hind legs. Hmm, my copy of Vaughn's book was stolen and they are now $500 on Amazon. Can anybody recommend a similar book on ballistics? See Mann's springs picture of spiral path. It explains a lot, to me. The ASSRA archives has Vaughn's book, and will loan it. I read it, hard, 3X. 270?

Mann's book at:

https://books.google.com/books/about/TheBulletsFlightfromPowdertoTarge.html?id=QdQqAAAAYAAJ

John