Home Forums Hunting & Shooting Competitive Shooting Tiny groups at 300; so so at 100? Paralax???

mcmurph, how do you argue the point that if you are in the center of your scope, there is no parallax?

Cant say I quite understand the gyroscopic precession model myself either, but that is more than has been offered so far.

How do you explain that 115g NBT out of my .25-06 will shoot .6s at 100 yards but will shoot .4s and .5s at 300 yards? I would attribute it to luck myself if I had not repeated this drill multiple times. Last time I went to the range, I shot 4 5 shot groups(clean barrel after 2 fouling) at 100y than went out to 300y after cleaning again and 2 foulers, repeated the 4 5 shot groups and the 300 yard groups were smaller on average.

I have an adjustable parallax scope on my rifle.

Good topic- keep them coming.

kyreloader;
My first instinct for an explanation is that it is a paralax issue, but I am perfectly willing to accept that it is properly adjusted.
I am perfectly willing to accept your group results.
What I am unable to accept is the explanation for the phenomenon. I cannot believe that a bullet can accurately and reliably turn an angle at some point in its flight.
As for Gyroscopic precession, it is about a gyroscope with a gimbal returning to it's natural axis after a disruption or initial instability. My issue with this explanation is that a bullet has no gimbal. Precession explains a bullet becoming stable, but does not explain it turning an angle in order to converge at an aiming point. http://en.wikipedia.org/wiki/Gyroscopic_precession#Torque-induced_precession

All this is to say that I believe the experience of the posters, I just don't know the explanation.

I agree whole heartedly. I am having alot of trouble understanding the discussion as well. I cannot understand how a bullet "goes to sleep".

I hope someone smarter than me can explain this to me and make me understand. I am willing to accept any readings available on the subject if anyone has any good links.

kyreloader, Gyroscopic precession perfectly explains how a bullet "goes to sleep", or more simply becomes stable. What it doesn't explain is how bullets turn and re-converge on a target.

The very BEST thing to do here, is quit WASTING time reading crap that won't make you a better shot, accept the paper results and keep on shooting!
IMHO, Jeff

Tonight is the first time that I have noticed this thread, but not the first time I have been in a discussion on this topic, what rost495 and Blaine are talking about really does exist. I have seen it several times myself.
I have never heard it refered to as "gyroscopic precession". I have always known or refered to it as "Yaw" or more specificaly "the yaw of repose".
I am not going to get in a debate with non-believers on this topic, if you dont believe go and buy a very fine book by Robert Rinker called "Understanding firearm ballistics". Robert has a chapter in this book that explains this topic very well.
B

boatanchor, I am sure that an under stabilized projectile can become stable down range, and physics explains this.
I am also sure that the better grouping my fellow posters are describing is also real.
My point is that if the groups are tightening up, after dispersing due to this yaw, physics demands a better explanation.

I have Rinker's book and dragged it out. The chapter describing "yaw" and "nutation" doesn't say that the bullet's path returns to some true path downrange. (Chapter 8 right?)

I have had this discussion with Jeff several times now, and don't doubt his observations. Another good book to read on this is Rifle Accuracy Facts, by Harold Vaughn. Dr Vaughn's says "poor statistics". I try to keep a more open mind on the matter.

If you have Franklin Mann's book, "The Bullet's Flight...", he first describes this spiral path, ingeniously documents it, then sets out to try to understand and explain it. He shows that the bullet does take a spiral path, but the center of the spiral is random once it leaves the muzzle.

I too would like a better explaination of the phenomenon.

I've never seen a group actually really tighten up, but I have seen the dispersion of the group basically stop for a while at mid range. IE same size group or just a bit bigger at 600 compared to 300 yards.....

Coincidentally, there's a parallel discussion going on in the NationalMatch forum. Here's about the most cogent and detailed analysis I've seen to date;
http://bryanlitz.bravehost.com/EpSwerv.html

I had the opportunity to pose the question about grouping smaller at longer range to distinguished Ballistic engineer, William C Davis Jr, of Tioga Engineering. He just wrapped up an incredible career doing work for outfits like Aberdeen with sidelines as a technical editor for American Rifleman. He's also the designer of the VLD bullet (Very Low Drag). In our conversation, he essentially asserted what Brian Litz says in his article. The "corkscrew" path is actually quite small (he introduced me to the terminology "swerve", and in all his work, had never seen the phenomenon in a controlled test.

What impressed me was that he didn't flat out reject the existence of the phenomenon, but just said he hadn't ever seen a well documented case of it. During the course of our conversation, he proceeded to think out how he would conduct an experiment to document the phenomenon using thin paper targets at various yard lines. Others since have extended that line of thinking proposing acoustic targets to further prevent disturbing the flight of the bullet, yet documenting the groupings at the various yard lines.

Great info, thanks Chris

I think this is the best communicated explanation. To me it says the rate of dispersion is descreasing slower than the increase in range. The bullet isn't turning back just dispersing at a slower rate than in the beginning of its flight due to it becoming more stable.
Does the same phenomenom occur with arrows? Seems that it should on a smaller scale.
Just some thoughts.

Battue

I got into a similar discussion over on www.longrangehunting.com and let me tell you, some of those guys are using higher math than me!
The simplest explanation I received is that the bullet spins around it's center of form in the barrel, then transitions to spinning around it's center of gravity after it exits the barrel. These two points are always a little different due to the inexact science of bullet construction.
The longer the bullet, the longer it takes to make the transition. During this transition period, the bullet is not stable and bullet paths are somewhat divergent. After stabilization occurs, the bullet path "straightens out", and the bullet paths are much less divergent.
The bullets do not curve back towards the center point, but they do not have to in order to have smaller MOA groups at longer distance. If we were talking strictly inches and not MOA, group sizes do continue to grow as distance increases. Sometimes not much after stabilization occurs, but they do get bigger.
I did have a guy claim that he saw a heavy barrel rifle that would shoot 3 inch groups at 100 yds but at 300 yds the groups were only 1 inch (yes inches, not MOA). I stopped short of calling him a liar but said that there was no way I would believe that unless I saw it--my diplomatic way of calling him an idiot.
I hope this explanation contributes usefully to this topic. I'm not an expert on exterior ballistics but it made sense when explained to me.

I haven't read through all the posts...I didn't need to. A bullet leaves the barrel...If it is spinning fast enough it will have little if no "YAW" so to speak..."same as a top that has been released from the string"...at first it will have large lopeing loops until it settles down into its "groove"...I'm not sure of the "physics" part but I've seen enough tracer rounds sent downrange to know that it can be seen...stable bullets will be pencil thin, whereas unstable bullets will have a "larger" looking glow..."SAWS" & fully auto weapons will have a larger heat signature than a comparable caliber in a low volume fire weapon...I imagine because the barrel heats up pretty quick & it takes a little bit of time for the bullet to "settle"...When first fired...full auto weapons will have a somewhat smaller signature than they will after a few hundred rounds are fired...As far as paralax is concerned...I have NEVER seen paralax as being an issue at 100 yds. then magically disappearing at two...It just doesn't work that way...

When I set as closely as possible the parallax setting on my scopes for 300 yds., I'll get groups from .87-1.5 inches with my rifles. But at the same parallax settings, I no longer get any groups much under .6-.7 @ 100 yds. But if I set the scope to be as parallax free as possible for 100 yds., I do get groups down into the .3-.5 inch areas.
It's all well and good to talk about placing your head directly behind the scope to eliminate parallax. I, for one, have never been able to achieve that. E

Yes, I'd say you have a parallax issue. I even dial it out when shooting ground squirrels with the lowly 22 LR. For young squirrels that are only 1/2 inch wide and a variety of shooting postions, it can have a signigicant effect when ranges vary from 5 to 150 yards or so.

I will not put a scope on any rifle that I expect to reach out that does not have some means of correcting the problem.

I just thought of a way one might weakly test this with a scope that does not have any parallax adjustments. Just hit paper at one range while actually sighting at a second. Lets say a given unit is parallax free at 300.

1) Take that unit and zero poi to be about 2 feet low at 300 yards.

2) This step will take some help... Place a target at 100 yards and a second at 300 that is postioned such that the 300 yd bull appears to sit precisely on the top edge of the 100 yard paper.

3) Fire 5 rounds while sighting on the 300 yrd bull. Since those rds are running low, they will punch the 100 yard paper while one is actually using a 300 yard sight picture.

If the unit is indeed a 300 yard nail driver, my bet is the 100 yard group will likely be a one holer too.

The reverse could also be attempted by running poi high with the same unit, sighting at 100 yds, and actually hitting paper at 300. If the unit has habitually opened up at 100, I bet it will really open at 300 while using the 100 sight picture. Something to do when we truly have too much time on our hands.

Still, optical or audio screens that have no physical effect on trajectory would be the primo test, but that would require more coins and equipment than I have available.

How much are the audio screens? Im kind of suprised a manufacture or someone with the necessary coin has not put this to rest with some quanatative results just to get their name attached to it.

To be honest, I don't know, because I don't move in those circles. My exposure is limited to conversations with some shooters that are just on the verge of moving into Olympic level competition. They have trained and competed in the Colorado Springs facilites where literally every aspect of ones actions can be recorded and played back. They have the ability to continuously monitor sight picture, heart beat, eye movements, etc, and link those with the nanosecond that trigger pull occurrs and the pill leaves the muzzle. One of my frustrations that I've not worked on but am aware of is a need to work within the tectonics of my heartbeat. I suspect I could halve group size with some training in that area alone.

Acoustic units are used around here in Oregon for some local/regional indoor stuff. Given the accuracy demands of competition scoring and the small targets, they must be quite dependable and precise. I think they use impact sound on a physical target though, detected by 3 or 4 mics, to triangulate shot placement. Thus, one does not have uninterrupted flight with an acoustic system. Their real plus was that scores of the whole line can be updated as shooters progress through their shots.

From a little surfing, I believe the next generation units are employing photo-electric or lazer scanner techniques that can monitor bullet flight at several ranges without affecting trajectory. To date though, I think they have only been used to track a single POI much like the acoustic units. With that kind of setup, the argument could eventually be put to rest though. Maybe I can take it on after retirement with my yet to be acquired lottery winnings.

A more costly issue might be the 500+ yards of extension cord needed to set up the computers and targets at distant ranges. Have a good one. 1Minute

There are acoustic targets somewhat within the means of hobbiests. Ken Oehler, who posts on this board in the "Ask the Gunwriter" sub board produces one that I believe is an option to his PBL package. As 1minute has mentioned, they use mic's to triagulate the passage of the bullet. Where it differ's from 1minute's account is that the shock wave is picked up by the mics and paper is not necessary.

I had a conversation with a noted Ballistic Engineer a few years back, and he proposed using very thin paper on the midrange target (ie onion skin etc) that would not disturb the flight of the bullet as it travelled to the terminal target. It's arguable whether that midrange target would appreciably alter the natural flight of the bullet. I'll have to go back and re-read Dr Franklin Mann's book too.