Home Forums Hunting & Shooting Long Range Hunting Smaller dispersion at longer range than at close range.

Brian Litz, ballistics expert is looking for folks that can help him document this phenomenon



Read about it here;
http://www.longrangehunting.com/forums/f19/applied-ballistics-shoot-thru-target-challenge-144359/

This happens to me from time to time.


When it does, the difference always lies with the shooter.

The parallax Phenomenon..

I could see it happening if a scope had a parallax problem at 100 but was parallax corrected for 200 yds.

1. Parallax.

2. Disturbance of bullet by release of gas at the instant of bullet boat tail exiting the crown. This is exacerbated by long bullets with long boat tails typically used in long range shooting and "magnum" cartridges that generate very high pressure at the muzzle as the bullet exits. Basically the release of extremely high pressure gas precursing around the long, barely stabilized projectile causes a bit of an "upset" to the gyroscopic stability in the first portion of the bullet flight.

Said another way; the bullet is getting smacked on the tail on the way out the door. Everything settles down once the bullet is out of the blast zone.

People have often referred to bullets "going to sleep" after a certain range....

I did not discover this nor have I collected the data that confirms. It was passed to me by a ballistician and bullet designer at a major ammunition company. I have experienced the phenomenon of less dispersion at further range many times.

Boddington has seen it,written on it. He pointed to #2 above as a cause as well.

Google "yaw", "precession", "angle of repose"and "nutation" as related to bullets in flight. (think I got those words right I was reading about it last night.

Those things (bullets) ain't always flying perfectly straight and nose on all the time

If it's a parallax issue another scope would solve it right? Do iron sights and target apertures have parallax?

As Bryan Litz points out, the phenomenon of bullets "going to sleep" doesn't explain how they somehow land closer to point-of-aim further downrange.

I'll be very interested in whether anybody takes him up on his offer of a free trip to his test-lab with such a rifle. It appears several people who've posted on this thread own one.

Physics, it aint for everyone...
It would take a bullet that has eyes...

The reason that I am skeptical about this is that in order for this to happen, the bullets that get pushed a little too far right would somehow halve to know to make a slight left turn when they settle down, the bullets that went left would have to know to turn right, the bullets that got pushed too low would have to somehow turn up, and so on.

Yep.

Bullets aren't like Thermos bottles that "know" how to keep cold stuff cold and hot stuff hot.

Some of the explanations floated so far such as statistics (a rifle will shoot some small groups and some larger groups, and it's likely you'll see some smaller groups when you shoot far, and some larger groups when you shoot close), parallax, and aiming error due to target size.

All these would be eliminated by Litz' test setup which would measure the path of the same bullets as they cross the short line, then continue on to print at the long line.

If you have some time on your hand, read the original thread (link in the original post here). I found it a good read.
C'mon folks, if you have such a rifle, you would be contributing to expanding the understanding of exterior ballistics...(and as Mule Deer had mentioned, would score a trip to spend time on the range with one of today's true ballistics experts)

Except that it is a physical impossibility. How people don't just take about 30 seconds and think about the thing is amazing. Once a bullet or anything has been launched at a certain angular deviation, it can't correct itself and become a smaller angular deviation. There are a multitude of reasons why people believe that they see groups getting smaller at longer ranges, but it isn't because the bullet "goes to sleep".


I've worked at a place with the same targets that Bryan is talking about. Of course just a bit of critical thinking would reveal that there is no way for a bullet to have 2 MOA say of angular deviation at 100 yards, but magically have 1 MOA deviation at 300 yards, but using those target systems and shooting through several of them simultaneously reveals of course that groups can only get larger as distance increases, not smaller.

The term entropy comes to mind.

Over the years, I've participated in many discussions about this phenomenon. The only credible bit of evidence that came up in support of the phenomenon was a posting by Gale McMillan on the old usenet recguns forum, which I believe was archived on yarchives. Mac described observing just such an effect while testing the 50 cal Sniper Rifles he produced for the military. I've searched high and low for that old posting recently, and can't find it now. Maybe someone with stronger Google-Fu than I will unearth that gem.

I forgot to add that there's another credible explanation for the effect manifesting in the vertical plane only called Positive Compensation.

BTW, I proposed acoustic targets when the Oeler's became available on the consumer market, but I think he's using old fashioned paper for the midrange.

You best drive your happy ass to Michigan!



Travis

What if the blast at the muzzle is not upsetting the stability of the bullet or its angle of dispersion but is just nudging the stable bullet off line slightly?

A perfectly straight launched bullet nudged over/up/down .001" would cause some of the "MOA shrinkage" at longer range. Something like that could cause a .75MOA group at 100 to go on to be a .5MOA at 300. Once the bullet is out of the blast zone there is no other force but wind and air pressure to act on it.

I have no idea.

I'm lucky just to feed mine the right ammo.




Travis

Chris,

Gale's post is at the bottom: Bullet helical flight path

Reading that prompted me to dig out my copy of Dr. Mann's book, where he performed the exact experiment as described by Litz. The results are as expected - linear - but the measured distances were short (100 & 175yds) and the number of tests small. Personally, I'd be listening to Gale McMillan on this kinda stuff.

Sure do miss recguns. The quantity and quality of info passed around from the likes of Bart Bobbit, Gale McMillan, etc. was just incredible.

John

ChrisF,

I suspect (but don't know) the reason Bryan's using paper targets is a lot of people wouldn't believe acoustic targets. Results from an Oehler 43 would be regarded much like posts on the Campfire that don't include "pitchers."

I am, again, amazed that the entire shooting industry hasn't tapped the Campfire as a resource. There are more confirmed ballistical anomalies and shooter expertize here than most magazines and books put together.

Aside from all that shared knowledge, I have managed to take stock rifles that don't print tiny groups and don't have fast twist, and still hit stuff. Color me skeptical...

Of course I don't know squat, and ain't much of a top notch shooter, but I can certainly tell you that without a doubt, the worked up 600-1000 yard load in my ARs with 90 jlks, will not group worth a flip at 300 yards. It wins matches at 600 and beyond. It looses matches at 300. So much so that I had to start carrying my 75 amax ammo to some matches that would cancel the full 600 yard stage if not enough folks were around, and shoot 300 yard reduced.... I did that a few times, and once in bad wind thinking the 90s would win the match for me due to really bad winds.... They did not. I changed ammo after about 5 shots.

I can say this, the ammo appears to basically hold a bit over MOA at 300, and very much so sub moa down to around .5 MOA repeatable over the course of many rounds and a many years of shooting this ammo. At least 5 years of shooting almost year round.

RE parallax... unless we had it with iron sights I can't blame it on that.

My theory has always been, not totally stable. Of course when working on 600 yard ammo, I started at 600... was suprised the first time I used it in a 300 yard match that it sucked really badly.

I have seen this same ammo many years before in a slightly faster twist, IE 6.5/7 twists the 7 would net you just the tiniest oblong hole at 100... but then I realized you are wasting your time developing loads at 100, when you are going to shoot them at 600, and promptly set up a 600 yard bench at my house.

While I have no "scientific" answer to this, I can tell you without a doubt to my eyes it exists.

And I could care less why, I just learned quickly where to use what ammo.

Paper has always been the final answer for me, though I"d include acoustical, but its about results, not what it "should" or should not do or calculate out to...