I bought a new 16" barrel DPMS carbine, my plan is to build on this. I would like to change the barrel out for a 20" what all do I need to buy to do this, I assume the barrel, gas tube, anything else? I bought this carbine because it was cheap and I cant afford the upper I would like so I am gonna do this project in steps. I have never swapped a barrel but I have a buddy that does this stuff, I just want to have everything together before a bug him. Any advice? Thanks.