It depends on the dia. of the barrel. What brake are you buying, and what gun is it going on? If it's a Beeman it comes with two different dia. sleeves. You pick the sleeve that slides over your barrel. Then put you brake over that and tighten the set screw against the sleeve.If it's a good fit against the barrel without using a sleevel, then yes, you tighten it against the barrel. Beeman brakes come with easily understood instructions.