yeah but.....
Tac forearms are shorter, and they allow the gun to more easily be unloaded from the mag tube by allowing simpler access to the shell stops. (this avoids the "bubba" and unprofessional habit of jacking all shells through the action and onto the ground when unloading )
Sporting 870 forearms are longer. Not sure why? Some of the vertically grooved wood factory forearms (seen on some LE/tac guns) are slightly longer too. The shortest are the aftermarket synthetic items.
Is it possible that your butt stock is to long, not allowing a natural reach to the forend?? The std length of pull factory butt is too much for a lot of folks, more so w/ any body geared up w/ armor and tac harness /vest.
As an aside, an neat trick is to attach a length of webbing fore and aft of the pump handle, forming a loop the off hand fits into when working the action. I wish I could say I thought that up myself, but I read the NYPD stake out squad did that in the '70's.