Have a 15(.223), how can I find out the twist rate. Stock barrel.

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It should be marked on the barrel.

You can figure it out with a cleaning rod. Push a cleaning rod into the bore with a brush or patch attached so the rod will spin. Start the rod until you can make a mark on the rod level with the muzzle. push the rod in further until the rod makes one complete spin and make another mark. Pull the rod out and measure the distance between the marks.

Keep in mind that barrel makers round off the actual twist rate. A 1:7 twist barrel might actually be 1:6.75.

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