This is what Ive been using for .177 bb.. but you can pick the ID you need. https://www.mcmaster.com/89495K275
https://www.mcmaster.com/89495K655/ This is the stuff you want for 1/4" rounds. It's 3/8 OD, pretty tight tolerance, and can be bent without needing cerrobend or anything as long as you've got a decent tubing bender. The 5/16" OD stuff is good for 7/32" rounds, and the 1/4" OD stuff is good for .177" bb rounds. I have not tested anything for 3/16" rounds, since I don't have anything that uses them.
