I have a Caldwell rock BR. I put a 3in.x1/2in sliding piece attached to the bottom of my stock. I tried shooting with a "thin" bag that came with the Caldwell bUT took it off and have just the hold down rails as roundediscussed Allen heads the Delton rides on. Do I need to be using a bag? Why is a bag needed on most rests I see like the SEB etc.?