1) diameter of the round part: .370"
2) length of the round part: .500"
3) distance BETWEEN BOTTOM OF BARREL to the bottom of Lug: .777"
Be sure to consult a picture closely -- there is a curved bevel at the rear and a incise cut on the front that will need to be duplicated. I'm imagining you have a Sporterized #5 that had its bayonet lug cut off but still has the base of the flash hider.
Good Luck