I've been looking at the Remmy 7615 pump action for my next rifle (I'd buy an AR-15 but....well,you know /ubbthreads/images/graemlins/mad.gif ). My question is, since the barrel is only 16" and looks pretty light, could it be swaped for say a 20" #7 contour? Or would the pump assembly have to be modifyed?