Ask Foghorn: Do groups “tighten up” at longer distances?

Tyler writes:

Have you done any research on bullet stabilization? Some of the boys back home seem to think shot groups sometimes tighten up at some point downrange. Have you heard or seen such a thing?

I was going to hold off on this one until I finished up the silencer reviews from my trip to AAC but it’s been sitting at the top of my inbox and starting at me for days now, mocking me, calling for me to answer it. And so I will. Grab your protractors boys, it’s time for physics!

Devoted readers of my technical jargon will remember that I did a little bit on why people use cheap ammunition to practice. In that article I introduced a concept called the “cone of uncertainty.” I believe my exact words to describe it were:

If you were to shine four lasers from the end of your barrel and through the furthest apart holes in your target (up and down, left and right) you would see an ever-expanding cone formed by these lasers. That’s the “cone of uncertainty” for your firearm and your ammunition; every round you fire will fly within that cone until it strikes a target. That cone exists even with bench rest shooting (where the firearm is in a vice and doesn’t move) because the variation is not solely a factor of your own personal accuracy, but also that of the firearm.

It’s an over-simplification of the concept, as gravity and wind and a number of other forces acting on the bullet tend to pull it in a parabolic trajectory rather than an actual laser straight line, but it works for our purposes. The reason that bullets form this ever expanding cone was first observed by a man clocked in the head by an apple. You may remember this little ditty from High School:

Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.

Or, in English, “a moving object will remain moving in a straight line unless another force moves it.” It’s Newton’s first law, one of the fundamental principles of physics.

When firing a gun, the expanding gasses of the gunpowder are the force making the bullet move. The barrel of the gun directs those forces in a straight line, usually towards a target of some sort. From the second the bullet leaves the barrel of that gun the shooter has no more influence on the path of the bullet, and it will continue in a more or less straight line until it hits something.

So what does that have to do with the question? If we apply Newton’s law, we see that there is no physical way for the bullets to suddenly “tighten up” at a further distance. The closest possible group you’re going to get out of a rifle is at the muzzle, and targets further away will always have larger groups. Always.

“But Foghorn,” I hear you cry like a high school student who thinks they’ve outsmarted the teacher, “other forces act on the bullet when it leaves the barrel! Couldn’t that change the trajectory?” Well Jimmy, they can and do alter the trajectory of the bullet — but they alter the trajectory in a uniform and consistent manner for each round fired. If one round you fire is pushed 5 inches left by the wind, the same wind is going to push the next bullet that much as well. In order for these external forces to “tighten up” a group at distance they would need to focus the bullets like a lens and affect them differently depending on their position, but instead they simply push everything equally.

So where does this myth come from? It’s possible that football (the American version) has an answer.

When the quarterback throws a football, you can sometimes see a bit of wobble as the ball starts its flight down field. The wobble disappears after a while, resolving into a nice spiral and landing exactly where he wanted it. When we say “a bullet flies like a football” we put that idea into shooter’s minds; that the bullet wobbles at the beginning of its flight. Which is not true.

The football wobbles because the force applied to the back of the ball is not uniform. As your hand releases the ball, the side of the ball with your fingers on it gets just a little extra “oomf” than the rest of the ball and causes the wobble. With a gun, the force applied to the bullet is constant and uniform thanks to the fluid-like gasses pressing on it from behind. When the bullet leaves the barrel there is no significant amount of wobble, and no time needed to stabilize in flight.

(As someone noted in the comments, bullets do in fact wobble when they leave the barrel. Slightly. This us usually due to concentricity errors made when loading the bullets, which increases the force on one side of the bullet in relation to the other side of the bullet and causes some minor wobbling. This wobble does alter the flight path of the bullet, but once it is corrected the bullet will not magically re-align with the point of aim. If there’s wobbling going on then the wobble will result in larger groups, not smaller ones, as the mean vector (the general direction in which the round is travelling) will not change. I didn’t want to go there, but as a commenter brought it up I thought I should mention it in the article proper. So there. I hope you’re happy.)

So, in summary, it is physically impossible for a group to “tighten up” downrange.

If you have a topic you want to see covered in a future “Ask Foghorn” segment, email guntruth@me.com.