I have a Colt lightweight barrel. I really have no idea how old it is. I'm trying to figure out what the rifling is. It is not marked on the barrel. Any suggestions on how I can do this?

Put a patch on a cleaning rod as though you were going to clean the barrel.

Make a mark on one side of the rod near the handle with a pencil and have it facing up.

Put the rod in the bore and put another pencil mark on the rod right at the point where the rod enters the receiver.

Push the rod through the bore allowing it to rotate with the rifling until the first mark is facing up again (one complete revolution).

Make another mark on the rod where the rod enters the receiver.

Measure the distance between the two marks.

This will reveal the twist.