Since you dont want anything on your chest I would use a battle belt with lots of molle. Then I would buy lots of pouches for pistol mags (works great for most sniper magazine) and an utility pouch for your back. If you need extra space for grenades and stuff just buy open topped M4 mag pouchs with pistol mag pouchs on the front. If you get a pistol just add it to the belt.
Some of my sniper friends use an MP5K on a rig on the leg or under the arm pit, lots of firepower