Gun Design: Accuracy

In my last post on Gun Design I wrote about pressure and recoil impulse.

This post will be about how to turn channel that energy into accuracy. There is a difference between accuracy and precision. For example in benchrest competitions where the group is on the target paper doesn't matter, it can be a very precise group, but not very accurate at hitting the point of aim (in fact some shooters deliberately adjust their point of aim so that their aiming point isn't destroyed by the bullets in the shot group).

Accuracy comes from consistency, and consistency comes from highly uniform ammunition in a firearm that can consistently shoot that uniform ammunition.

There has been a lot written about accurate rifles, and it is my experience that barrel harmonics are quite an important part of the equation. Using a heavy barrel bolt action rifle chambered in 308 Winchester with a 1:10 barrel twist I can change group size from almost 8 minutes of angle to 3/4 MOA simply by changing the ammunition. For example Federal 180 grain commercial ammo does not shoot well at all through my rifle, they group horribly at 100 meters. But 168 gr match ammo shoots tight.

It isn't that the 180 grain ammunition is bad ammunition, far from it. What happens is that the 180 grain bullet does not leave the muzzle of my rifle in a consistent manner. This is because the bullets are not leaving the bore at the same time, same velocity, or both.

When the powder in the cartridge is ignited it creates pressure, a lot of it. This pressure presses outward like a hammer blow to the steel chamber, and this sets up vibrations and resonance throughout the barrel (and consequently the entire firearm). While the pressure is pushing the bullet down the bore, it is also pushing the rifle rearward, sometimes as much as 1/16th of an inch before the bullet leaves the bore.

The commercial Federal ammunition did not consistently set up all the movements in a consistent manner in my rifle. My rifles barrel is "stiff" because it is massive, and this means that the oscillations of the muzzle will be smaller in magnitude than a thinner barrel of the same length. This means that both muzzle oscillation and bullet velocity were involved in the large group size.

Imagine the nozzle a firehose moving free as it sprays water, it isn't entirely chaotic. The muzzle of a rifle does the same thing as the bullet travels down and out the bore. The stream of water has a different arc depending on angle and velocity, and if someone was turning off the water the pressure would rise, and in order to keep the splash on target the angle has to get higher.

Up until now I have focused on ammunition consistency, now I will go on to rifle consistency.

Consistent final lockup. This means that the rifle positions the bullet properly in the chamber, aligned so that the bullet enters the barrel throat without massive deformation.

Consistent trigger. The trigger needs to be break at the same time every time. Some shooters don't want to know when the trigger breaks, but the trigger should break with the same force applied each time.

Consistent ignition. A fast lock time is not as important as consistent ignition (the AR15 has a slower lock time than the m98 action). The firing pin needs to hit the primer the same way every time so that the primers can perform consistently with each other. This means the closer to center the better, and a consistent hit strength. Having an out of round firing pin channel and hole can cause the firing pin to hit at a different position on the primer. Having a bunch of grit and crud on the firing pin can cause the primer to hit with a different strength and velocity.

Consistent harmonics. The "stiffer" and "tighter" a rifle is the less variation it will have. This means tight bedding for the action (or even a barrel block for some rifles) so that there won't be any shift between stock and metal. This also means that shorter stiffer barrels are going to shoot tighter than longer barrels of the same diameter.

True muzzle crown. The gasses that fly out of the muzzle behind the bullet need to fly true so they don't impart yaw and make the bullet fly less than true.

Add it all together, consistent ammunition, good trigger, consistent lockup and ignition, tight harmonics, true crown and you should have a tight shooting rifle.

There is a lot more to talk about, but these are the broad basics that accuracy is built on.