Don`t buy a 20" barrel
They reckon anything past 14" starts to be a waste, you won`t see a massive change in accuracy but you will be wasting more air.
I have a 16" one piece on my tippman98 which helped with its accuracy massively against its stock barrel, which is tiny, like an 8"
But even that feels ridiculously long, a 20" barrel on a proto SLG would be OUTRAGEOUS it would feel like your holding a spear.
Go with a 14" they are most effective in all aspects, preferably a two piece as its easier to clean broken paint and get that oil out of the barrel which is very hard with a one piece.
Sorry i`ve moved away from the thread topic, but I couldnt see you go buy the wrong thing!