Try the cleaning rod method. With the bolt in the upper, insert a cleaning rod with a flat tip from the muzzle, and mark the rod at the muzzle when the cleaning rod hits the bolt, and retract it 3 or 4 inches. Now remove the bolt, insert the bullet and lightly push it up to the rifling with a short rod. Now carefully push the cleaning rod back towards the chamber till it touches the bullet, and measure from the muzzle to the mark you made on the cleaning rod. You can use tape to mark the cleaning rod.