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.