Ar15 add-ons

Ok! so im onto the next dilemma...

i have a 20" upper rifle... i want to upgrade from the fixed A2 stock to the Magpul ACS stock... what buffer tube/buffer do i use?

already planning on running a sprinco Blue or red buffer spring and 5 coil extractor spring...

its just all the tubes i find are "Carbine" tubes...

am i just getting hung up on terms? is the 6 position UTG mil spec tube and nut fine, and then the sprinco springs, with stock buffer?

im so lost...