I use an aluminium welding rod to measure the twist rate, flatten one end so it is a very snug fit across 2 rifling grooves, then cut it down to about 20" long and bend 2" up at the opposite end. Feed it into the barrel until the sticky up end is either horizontal or vertical, measure then move it in or out until it has turned 180 deg and measure again, take one from the other and double it. Unless you have a very long barrel you wont see a full rotation.
Hammer forged barrels could have a variation in their twist rate from different batches as the barrel grows in length 10-15% as the metal is displaced longitudinally while being forged down onto the mandril, so machine setup could cause different growth and therefore the length in relation to a full twist.
HFT, for people with only 2 buttucks!
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