No, only bone structures within R15 humanoids use Inverse Kinematics, if that helps you can replicate an R15 rig to do so. As for regular parts it can’t be done.
Technically yes it can be done, but it is going to be very difficult and I’m not sure where to start. What you need to do is load the player animations, model, and gun model into blender and use the built in inverse kinematics functionality there. Then bake it into your animations and reload them into Roblox.