Bullet Dispersion

[ptosis]

mman, you are absolutely terrific!
Being a long-time lurker, I have registered on the forum just to say thanks for this one!

You have done a dramatic job of debunking one of the most tenacious myths of long-range shooting, and done so with ease, grace, and legibility.

Kudos!

 

[dcali]

If anyone's interested, the page in question in Vaughn's book is 170, which does in fact give bullet deflection at the target as being linear with TOF.

That's in contradiction to McCoy's book, which goes through much detail to come up with deflection due to lateral throwoff and aerodynamic jump (aka bullet imbalance) as being linear with distance.

In Vaughn's defense, he hints that the equation was pulled from Franklin Mann's book (which is an a great piece of history in its own right), and that it was in fact based on incorrect reasoning. But he doesn't get into detail (which would be way over the heads for Vaughn's intended audience).

 

[BoydAllen]

Getting back to the part about loads that shoot well as short range not holding up at long range, there is another factor that seems to have been ignored, twist. Some time back, I read an article that was about the perception that .308s are more accurate than 30-06s. Hart barrels in the standard twists for each were rechambered into both calibers, and shooting tests were done. Without boring you with too many details it came down to this. Most of the testing of rifles is done at 100 yd. and at that distance with the same long range bullets, the 12 twist (standard for .308) had the advantage, regardless of which of the two cartridges that it was chambered for. At longer ranges (600 yards if memory serves) , same bullets, the 10" twist (standard for the 30-06) gave better results than the 12. I believe that the article was authored by C E Harris, and published in American Rifleman. None of this is to cast doubt on anyone's results, but rather to broaden what factors are considered when trying to explain what happened.

 

[dcali]

Not only twist, but bullet geometry. When you untangle the math, the sensitivity of a bullet to imbalance is higher for longer/skinnier bullets than shorter/fatter bullets (an oversimplification, but basically the way it works out).

In other words, those short stubby benchrest bullets aren't just better made, they're less sensitive to the problem to begin with.

 

[smokey4570]

In Vaughn's defense, he hints that the equation was pulled from Franklin Mann's book (which is an a great piece of history in its own right), and that it was in fact based on incorrect reasoning. But he doesn't get into detail (which would be way over the heads for Vaughn's intended audience).

Mann's equation used range:

E=36*pi*R*d/P