I have long wondered this , you CAN put an additional/aftermarket upper ( say even a diff style 556) on the R-15? I like the R15 always have , And I believe there is a VTR PREDATOR CARBINE that i love as an SPR- 18" bbl, free float tube foreend , flattop receiver , mmm mmm- and to throw a black CQB receiver once in a while even better n