If your going to want to swap you'll need two uppers. There no quick switching bbls like in the movies. Get a 16" and you will see that you have no need for an 18". The amount of added accuracy and velocity that 2" will add is negligible to all but an expert shooter. Carbine courses are going to be full of guys using 14.5s. Your not going to see anybody with an 18" bbl in those courses.