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It's in the FAQs and possibly the link supplied.
But all you need is a cleaning rod, jag, patch, tape and a ruler. Incert tight patch into the bore. Mark rod at junction with breech. Push it in watching for one complete revolution of the rod. Stop at one revolution, mark rod, remove from bore and measure. One twist in ??" between the marks is what your barrel is.
The faster the twist the better it will stabilize longer bullets. Bullet Weight is irrelevant. It's simply longer bullets are generally heavier.
CW