The G&P M870 has been highly customized recently but only with the externals. You can never go to far with stylizing your M870 to make it look as amazing and different as possible.
There is where the Forearm set comes in. The highly customized 230mm outer barrel and a tri rail pump action grip that goes with it and it's all made out of metal. Since it is something you grip when you pump the shotgun, it'd be easier to get a vertical grip for it (which does NOT come with the set). As for the others other 2 rails, you're able to place any accessory you wish!