How does one measure the twist of a gaintwist barrel ?. I have aquired a used RKS barrel mounted n a 44  1/2 frame and the rifle shoots just fine but for my own satisfaction I would like to know what  and or be able to measure the twist of the barrel . Thank you

Thank s for the reply, now that is going to give me  something to ponder on .

I have seen the mechanism that changes the single point cutter's twist as it progresses down the barrel.

Ron Smith duplicated Harry Pope's mechanism.

A cable on a spool where the angle of the cable changes by moving the placement of the end further up the wall. 
This changes the rate of twist; increases the twist, as the cutter moves down the barrel. 
By how much is dependent on the angle of the cable on each side of the spool.

Simple mechanism controlling the cutter but I cannot explain how it changes the gain twist as it progresses down the barrel.

Ron had a table listing the start and finish twist depending on where the ends of the cable were located resulting in the angle of the cable before and after the spool.

But very difficult afterwards to measure the twist of a section of the barrel but Ron's table has all the data as to what the start and finish twist is.

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My .32 RKS 8 groove barrel starts at 20:1 and ends at 11.5:1.

The high power long distance competition shooters burn out their barrels and need to replace them annually.

Using RKS gain twist barrels the high power shooters have confirmed they get a 10% to 15 % increased life span over using a straight twist barrel.

A gain twist barrel does not enhance or degrade accuracy. 
It only has a longer lifespan.

But for single shot rifles with a normal lifespan of at least 300,000 rounds many Schuetzen shooters probably do not care.

A gain twist barrel accelerates the rate of spin going down the barrel; whereas the straight twist barrel it has to jump to the fastest spin rate immediately upon ignition.

Does a faster jump start in a straight twist mean more lead can be sheared off which starts leading to occur sooner?

I am aware of a fellow who installed a gain twist backwards on his rifle and found it did not gyroscopically stabilize since it ended up decreasing the rate of spin to the point the bullet tumbled.

My only experience with gain twist vs straight twist is in Civil War muzzleloading artillery. I've shot target competition with my 10-Pounder Parrott Rifle since 1993. Many of those matches were on a military base with 4' x 6' targets at 1000-1200 yards.  The 3" Ordnance Rifle and the later (3" bore) 10-pdr Parrott shoot the same projectiles and the same charge of powder (1 lb FG GOEX) The Parrott has 3-groove gain twist rifling. The Ordnance Rifle has 7-groove constant twist rifling, 1 turn in 11 feet.  The final twist rate of the Parrott was 1 turn in 12 feet.   

Handled by equally-experienced gunners using identical projectiles, firing on the same range, under the same conditions, the Parrott will out-shoot the Ordnance Rifle most of the time. 
Proves very little I suppose, but it is my experience from 30+ years of shooting.   

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To sort of reflect on what John Taylor wrote, I am thinking most of us are shooting rifles with "gain twist". As the bullet accelerates down the bore the RPM's are building until the bullet pops out the end. The exception would be if the barrel is too long and pressure is not building throughout "barrel flight" but rather beginning to drop off because the propellant has been completely consumed and friction in the bore is causing the bullet to slow.

Another lecture  . The purpose of a gain twist is to minimize the peak torque on the bullet. Peak torque is a function of acceleration, not velocity.
For a uniform twist, the bullet torque curve would look much like the pressure curve of the powder burning. Peak at the breech and decreasing toward the muzzle.
Minimum torque is achieved by applying a constant torque to the bullet from breech to muzzle. Minimum torque-maximum acceleration time.
This technique can reduce the peak torque on the bullet by approximately one half. This would vary based on the pressure curve of the powder.
A straight line twist rate change of a little less than 2:1 will approximate a constant torque on the bullet.
That said: Torque required to spin a bullet of rifle calibers are so low that they are easily resisted by the bullet.
Gain twist appears to be a wonderful solution to a problem that does not exist for rifle calibers. Lower forces would increase barrel life.
Chuck

I have 1 rifle with a gaintwist barrel, and it might be the most accurate .45 BPCR I have.  Could be coincidience, of course.  It is used shooting PP bullets, that might also make a difference.