PURPOSE: To reduce an air resistance of a projectile by a method wherein a separating type lag engaged with a rail and slid rearwardly and disengaged from a projectile is installed.

CONSTITUTION: A front part separating type lag 1 is slid rearwardly under reaction of firing of a lag fixing fitting (a chemical bolt) 5 and air resistance received at the front separating type lag 1, the lag is disengaged from a front lag pressor 3, transmitted along a rail 6 and strikes against a rear part separating lag 2. The rear part separating type lag 2 is slid rearwardly under a reaction generated when the front part separating lag 1 strikes against the rear part separating lag 2 and air resistance received by the rear part separating lag 2, disengaged from the rear part lag pressing part 4, slid on the rail 6 and moved rearwardly. Then, it is disengaged and separated from the rear end of a projectile 04. With such an arrangement as above, air resistance of the projectile 04 is reduced and a flying performance of the projectile 04 can be improved.