First of all, you should adjust a weapon collider.
Then, go to the weapon prefab and adjust the position of "AttackSpawnPoint", "ShellSpawnPoint", "AttackCollider" (depending on the attack in your weapon).
And after, go to the adjustment scene.
To adjust a weapon with the IK system, you should open the adjustment scene [Window -> USK -> Adjust], then place your weapon or a few to the Adjustment script, go to the Play Mode and choice the weapon there.
Read about the further adjustment there:
Or see the videos: