I too used to think the idea of a 'magical' bullet that, like a heat seeker, gets more accurate as it flies towards the intended target was complete BS. I mean, how can a bullet not only change it's path in flight, but selectively and consistently do so in a way that makes it more accurate over distance? OK, I still don't think there is such a thing as a bullet that gets more accurate over distance in a 'heat seeker' manner, but I found a theory that I could get my head around.
Hard to explain in words, but here goes... I envision the bullet coming out of the bore spinning rapidly, and as Fifty Driver put it, "try(ing) to spin around its center of gravity. This is seldom exactly the same point as the axial center of the bore. As such, there is some "wobble" ... in the bullet" as it flies. I see this as the bullet 'cork-screwing' around a central axis as it flies towards its intended target, not 'cork-screwing' out of control, but at a fixed, or semi-fixed, amplitude (or period). This is similar, as Kirby said, to a top spinning on its axis, but also the tip of the top moves in a circular motion with reference to a given point on the floor. So now lets assume that the 'central axis' about which the bullet revolves (or cork-screws) as it flies through the air is a straight line that represents the 'angular' accuracy of the rifle. Lets also assume, for the sake of an example, that the amplitude (or total diameter) of the bullets 'cork-screwing' path around the central axis of the bullets flight is 3/4 of an inch and that the rifles angular accuracy is 1/2 inch. With that, one could expect groups at 100 yards to be 1/2" + 3/4" = 1.25", or roughly 1.25 MOA. At 200 yards, by this model, one could expect groups to be (1/2" x 2) + 3/4" = 1.75", or roughly 0.875 MOA. At 500 yards, (1/2" x 5) + 3/4" = 3.25", or roughly 0.65 MOA. At 1000 yards, (1/2" x 10) + 3/4" = 5.75", or roughly 0.575 MOA... Get the point?
This example does not even make the assumption that the bullet 'settles down' or 'goes to sleep' as it flies, but that it maintains a consistant rate of 'wobble' or 'cork-screwing' around a fixed linear axis as it flies. By this theory, the apparent increase in MOA accuracy would be even more extreme if it was assumed that the bullets wobble settled or went to sleep over time and distance.
Any way, it's is just a theory, but it is how I was able to get my head around the Phenomena of increased MOA accuracy over distance.
To really make things interesting, consider the effects this has on BC!!! That will really get you scratching your head.
I have seen many cases with bullets that have BC over .7 and especially those with BC in the .9 to 1 range drive to high velocity. The BC within the first 300-500 yards is often notically lower then the BC from 500 to 1000 yards or so. Then you often see the BC drop again in many instances past 1000 yards which can really make developing a drop chart a challange.
Exbal really allows you to tune your drop chart to your actual trajectory. Just a plug for Shawn Carlocks new video, get it and you will learn how this is done properly.
I also understand this to be the case. The disruption in flight created when the bullet transitions from rotating on the axis of the bore to the axis of the bullet will reduce the dynamic BC of the bullet. When you find a bullet/barrel combination with minimmal disruption from axis to axis (i.e., a fast transition) you have found what famed BR shooter Tony Boyer called a Hummer - a gun that seems to cheat the wind. The dynamic BC is higher than expected because the bullet stabilzes sooner.
To borrow the football analogy, anyone who has thrown a football knows that a ball thrown with a good stabile spin when it leaves the hand with go further and straighter than the same ball thrown with a wobble that stabilizes at 20 yards. Wobble is not aerodynamic.
The issue Kirby brings up regarding the bullet spinning on its own center of gravity is something I heard in 1990 in Tucson at the range. That is why bullet concentricity is so important. It's also the reason I like Barnes bullets. I am inclined to believe that if there is one solid shank of metal (bullet), its center of gravity is truly its center point axis.
Not the best bullet for long range, but inside of 900 yards, they seem to kill game just fine.
Out to 900 yards? I think 900 yards is long range by most anyone's definition. Is by mine.
Mine too, but "long range" is subjective per shooter. I read where many of these talented guys are hitting gongs and game at 1200+ yards. That is beyond my ability here in Ohio as I have no where to even practice beyond this.
My personal "long range" is anything over 600 yards.
My point was that Barnes bullets are controversial in terms of their published BCs, and ability to open up at lesser velocities at certain ranges, taking into consideration, MV, caliber, etc. By our accounts, they work just fine on antelope at 903 yards.
Another interesting point to me is that down here around sea level we have not seen it as described..... And, I guess I am glad I don't have to deal with it, at least not as of yet. We may just have been lucky over the last five years to never come across it. To me it is much, much, much easier to develop/test loads (powder type, powder charge, bullet seating depth, neck tension, etc, ect) at closer ranges and then confirm them at longer ranges. So far we have never had to go back to the drawing board for more data or to re-perform the diagnostic testing due to poor findings at longer ranges. All of our loadings in different barrels, guns and shooters have been spot-on at longer ranges when the load was tuned for peak accuracy and absolute minimum velocity deviation at closer ranges (ie 100 yards). Some barrels will shoot better than other ones and some reamers cut better chambers/throats than others, but we still have never had to can the data and start all over again.
The longer the range, the more variables that cannot be accounted for that could/would influence decisions for that particular cartridge. I personally think that it is somewhat altitude dependent/barrel twist dependent. Has anyone tried to "over-stabilize" a projectile for a given altitude and see if it settles down earlier than the standard twist for the same size projectile?
FWIW, we shot some 1.829" 218gr Aluminum tipped .30 caliber bullets out of the 30/378 Krieger barrel the other day to test stabilization for that bullet and they were very asleep at 100 yards as indicated by the group that you could not even stick a .338 bullet into. Even better the bullets went into the same hole with significantly different powder charges but with the same seating depth.
I feel your pain about the wobbling bullets at 100 yards, but feel very blessed to not have to deal with it. I guess that when you develop loads at longer ranges due to the stabilization issue (due to the wobbling bullet), that you just have to accept the accuracy-robbing variables that cannot be accounted for and also have to prematurely curtail load develpment and tuning prior to the peak accuracy load and tune being achieved.
I've been lurking around these forums for some time now and have read a lot of amazing, very useful information! I am getting ready to work up a load for my 7mm RM with the berger 168's and I had a question about something I read.
I've read in several places in these forums that one must sight in for at least 300 yards so that the (high bc) bullet will have a chance to stabilize. I read that at 100 yards you can have 1 or higher MOA but that at 300 it could get better - or visa versa.
My question is: How can this be??? Your bullet does not have fletchings on it like arrows. I always thought your bullet is either stabilized right out of the muzzle because of the right twist rate in your barrel (and the quality of your barrel), barrel harmonics (the right node), or it's not. Either the quarterback throws a lame duck or a tight spiral - not a lame duck that suddenly turns into a tight spiral.
How can a bullet that is on a plus moa path at 100 yards suddenly be .5 moa at 300? It can't just change directions from it's established course. I understand sighting in at 300 yards (and 800 plus for that matter) for reasons of validating your trajectory or measuring extreme spreads, etc. But allowing your bullet to stabilize???
Can anyone offer an explanation based on real ballistics or physics?
In a perfect world, it would be BS. But we don't live in a perfect world. Bullets are not perfect. Too many trusted names in the shooting and gun writing biz have noted this phenomenon. And it is a phenomenon that, is largely unexplainable via science, or explained like that above when the bullet's axis isn't quite centered with the bbl's. They refer to it as the bullet "going to sleep". It finds it harmonic perfect balance. This has been well documented with the 50 BMG over long ranges, and with many other long range guns. It just is.