I'd like to hear other methods myself.
   
I used a degree wheel that's used for setting cam angle on an engine, a rod a little longer than twice the length of the barrel, with a needle attached to the rod for measuring degrees, wrapped the rod with masking tape and marked every inch - I cast a short plug of plastic and pinned it to the rod so as the barrel was moved up and down the rod, the rod was forced to turn.  I made a chart and plotted all the points corresponding with degrees.  With the help of Excel it gave a very clear twist rate per inch...   FWIW - it also showed that some button rifled barrels aren't always as consistent as we may believe, they stutter, they accelerate, a few are what they say they are -  the thrust bearing was in good shape? 

I'd like to think there's an easier way but I can't visualize it.
Greg