To stabilize bullets in flight we must spin them. Here’s a barrel that’s been cut in half by our custom firearms shop at the Army Marksmanship Unit.
You can see the interior of a barrel on one of our competition AR-15s. You can see the round as it sits in the chamber, and you can see the rifling inside of the barrel, which causes the bullet to spin as it passes through the rifling.
You can also see the gas port, which causes the gas to leave the barrel and cycle the weapon.
Here is a recovered bullet and you can see the engraving, or the rifling marks on the bullet, which were caused when the bullet was pushed through the rifling, causing it to spin very quickly as it leaves the barrel. If the bullet is spinning at the correct rate of speed, it will be stable in flight.
So how can you tell if you have the correct bullet and barrel combination? I have a quick test to show you. We have here one of our M16 A4 Competition Uppers.
To do this test, you’ll need something to measure with, a cleaning rod with a tight-fitting patch or bore brush, and a piece of masking tape.
I’ve attached the masking tape to the cleaning rod so it points straight up, or at 12 o’clock.
As I pull the cleaning rod to the rear, you’ll see the cleaning rod spin, which is caused by the rifling.
So I pull it to the rear until the tape is back at 12 o’clock, showing that I’ve achieved one full turn.
As I pull the cleaning rod to the rear, you’ll see the cleaning rod spin, which is caused by the rifling. So I pull it to the rear until the tape is back at 12 o’clock, showing that I’ve achieved one full turn.
Now, I measure the distance that it took it to make one full turn and, as you can see, the rate of twist on this rifle is approximately 7-¾ inch, which is correct. Now that we know this rifle’s rate of twist, we can determine an appropriate bullet to fire for it.
Fortunately, most bullet manufacturers will tell you the rate of twist required to fire their bullet out of your barrel. This box of bullets says that a one-in-eight-inch-twist, or faster, is required.
Since we measured this barrel’s rate of twist at one in 7.75 inches, that is a faster, or tighter rate of twist than one-in-eight. So we should be safe to fire this bullet out of our barrel.
Additionally, there are some good reference materials out there that show you the rates of twist required to fire bullets out of rifle barrels, such as this, Ballistic Performance of Rifle Bullets. This gives most of the bullets we currently use and the rates of twist required.