It is likely you will have a better result using the "Create Cranks" command of the Kinematics Editor to define the gun Kinematics. Once the kinematic structure is correct, assign the geometry to the links.
I can get exact mechanism with RPRR linkage mechanism and i can get j1 as the main joint for gun opening but the opening value which is i am getting is -70mm max. but as per above mentioned method i am getting value for same j1 as -140mm. So i am not able to understand why this difference is occuring?