I have measured a lot of chokes and bores over the last 30+ years, and have used spring leg calipers to transfer the measurements to micrometers, inside dial calipers and CSP bore and choke dial gauges. All work, but the CSP gauges are the easiest to use. I have measured a number of M42 skeet chokes, and all were 0.005" constriction based on bores of either 0.409" or 0.410".All the Remington .410 skeet chokes that I have measured in 11-48, 1100, 870 and 3200 guns were also 0.005" constriction. I own or have owned in the past a number of o/u guns with Purbaugh and Briley sub-gauge tubes, and all of them measured 0.007" of choke in the .410 tubes (fixed choke). I much prefer the 0.007" constriction because I like the way the targets break better.
