Measuring a Barrel's Twist Uniformity

In our last post, we discovered how to measure the twist rate of a barrel. Measuring the twist rate of the barrel is one thing, but how do we ensure that the rifling inside the barrel is uniform? We will study that process in this post.

Back when we studied different methods of rifling barrels, one of these methods was called Button Rifling, where a tool is pulled or pushed through a barrel to create the rifling grooves. One of the problems we noted at the bottom of that post was that if the button slips inside the barrel while it is being pulled or pushed through it, the grooves may be non-uniform. So even if the twist rate is (say) 1 turn in 10 inches, the rate of twist may not be uniform throughout the 10 inches of length. So how do we verify the uniformity of the twist throughout the barrel.

The solution was an invention called the Twist Deviation Machine invented by Mr. Manley Oakley of Seattle, WA.

Mr. Manley Oakley, inventor of the Twist Deviation Machine

Mr. Oakley was a world-class benchrest shooter and had won the National Bench Rest Shooters Association (NBRSA) trophy several times. He was a well known figure in the Northwest chapter of NBRSA and they now award an annual trophy in his name.

Mr. Oakley's theory was that if a barrel had a uniform twist rate, or the twist rate slightly increased as it approached the muzzle, all other features being normal, the barrel would be an accurate one. On the other hand, if the barrel's twist rate changed non-uniformly (e.g. speeded up and then slowed down, or vice-versa) or if the twist rate decreased towards the muzzle, the barrel would be less accurate. He verified this theory by testing barrels that were known to be accurate against other less accurate barrels of similar physical characteristics. In order to determine the uniformity of a barrel's twist, he invented the Twist Deviation Machine to measure it.

It consists of a hollow steel tube with a thinner steel rod passing inside it. The steel rod is free to rotate inside the steel tube. To the steel rod is attached a plastic washer and about three inches away from this washer is a second plastic washer attached to the steel tube. This apparatus is then pushed through the barrel. As the washers are being pushed through the barrel, they engage the rifling and start to rotate as they are pushed through, which results in the steel rod and the steel tube rotating as well. Now if the rifling is uniform throughout the barrel, the rod and tube will both rotate at the same rate. However, if the rifling is not uniform, then one of them will rotate faster or slower than the other and this can be easily observed by looking at the parts of the rod and tube that are sticking out of the barrel. This allows the user to measure if the twist rate is increasing or decreasing through the whole length of the barrel.

Measuring a Barrel's Twist Rate

When we studied the basics of rifling a while back in this blog, there was mention of a term called "twist rate" for a barrel. The twist rate is defined as the length of the barrel required for a bullet to make one complete turn of the barrel. For instance, a standard M16A2 barrel makes 1 turn for every 177.8 mm. of barrel length. It is very important to match the twist rate based on the weight, diameter and length of the bullet to ensure accuracy. We studied a couple of methods on how to calculate the twist rate for a given bullet using the Greenhill formula and the Miller Twist Rate formula. All this is good in theory, but how do we actually measure the twist rate of the barrel? This post discusses a simple way to do so.

Recall that about 16 months ago, we discussed various tools used for cleaning firearms. Well, people can use some of those very same tools to measure the twist rate of a barrel. All that is needed is a standard cleaning rod with a rotating handle, a jag and a cotton cleaning patch. The user simply attaches the cleaning patch to the jag end of the cleaning rod and then pushes it into the barrel until the cleaning patch engages the rifling of the barrel. The user then takes a piece of sticky tape and attaches it around the back of the cleaning rod near the handle like a tiny flag (or uses a marker and marks a spot on the cleaning rod). The user then measures how much of the cleaning rod is sticking out of the barrel (a ruler measuring the distance from the barrel's base to the start of the flag or mark point should do it). Then the user pushes the cleaning rod into the barrel. Because the cleaning patch has engaged the rifling, the rod rotates as it is pushed into the barrel. When the flag or mark point has made one complete rotation, the user then measures how much of the cleaning rod is sticking out of the barrel again. The difference between the two gives the twist rate. For instance, if the rod initially has 22 inches sticking out the barrel when the first measurement is made and 12 inches sticking out of the barrel when the second measurement is made, this means that the twist rate is 1 in 10 inches.

For those of you who would like to see a video of the process:

Happy viewing!