Love this mod. Two things though : First, shouldn't it function the same as the Remington M870's (locomotive/Reinfield) shell rack, increasing the magazine while reducing the concealment?
Second, shouldn't the Winchester 1887 (Breaker 12G) have a shell rack as well? It's a shell loaded shotgun, however I can see the problems with attaching one to it. The mod "Extra Attachments Compilation", for example, has a shell rack which attaches to the extended stock only.

When I think about it, it's hard to figure out what the shell rack should actually do. The Overkill way of increasing the magazine doesn't make much sense when the shells are never actually consumed nor are they inside the shotgun. Increasing the ammo capacity or increasing the reload speed does make more sense I suppose. If it increased the reload speed, would it be like a speed pull mag but for shotguns?

Those are just my thoughts, thanks for the work that goes into this.