In the past the school of thought was to have a piston's second ring gap specification was to have a tighter gap than those for the top rings because they didn't see as much heat. Unfortunately this didn't account for inter ring gas pressure build up between the top and second piston rings. If the pressure between the top and second rings equals or exceeds the pressure above the top ring it can cause the top ring to lift off the bottom of the piston ring groove and lose contact with the sealing surfaces. This also inhibits the piston ring's ability to transfer heat from the piston. In order to keep the inter ring pressure from becoming a issue, the current trend is to create an easy path for the built up pressure. This is done by gapping the second piston ring larger than the top ring and a additional benefit of the increased piston ring gap is that the gas pressure is now directed downward towards the engine's oil sump and any oil that has collected in the ring area will go with it.
A piston's ring groove design is far more important than it may appear to be. Properly designed ring grooves have a small degree of vertical uplift, which compensates for uneven temperature growth of the piston as it reaches its operating temperature. Ring groove smoothness is likewise extremely important, any unevenness or roughness can cause a poor piston ring seal that can lead to micro welding, a destructive condition where under extreme pressure the rings momentarily attach themselves to a high spot on the ring groove.
In addition there also should be a small radius where the vertical and horizontal portion of the ring grooves meet. Pistons without this radius are more prone to groove distortion and ring land breakage.