mark II target question

Found an clean (1983)prefix 18 bull barrel at a yard sale today. Question is where do I measure the barrel length? From the edge of the reciever or at the back of the chamber? From the outside it is 4" - from the chamber it's over an inch longer.

Thanks
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YOU measure from the beginning of the chamber;ie: drop a cleaning rod down the muzzle to a closed bolt, mark it, and measure it . THE HB guns were 5 1/2" plus or minus a furlong or so.

Thank YOU.

It sure feels handy all balanced..perhaps a furlong more and it would be a chore to shoot.

44