Dust
As you know the common pump / autos have a 4 round mag. From your post you want a higher mag capacity. Remington pumps & autos are easy to find extensions for.
http://www.brownells.com/.aspx/pid=4...ZINE-EXTENSION
People doing 3 gun matches like high mag capacity, so search for their ideas. You are going to seriously alter the swing dymanics of the gun by putting on an extension. This works ok for shooting stationary targets, but not so good for aerial fast moving targets.