it's going to be interesting to see how you are going to capture the BBs traveling down the barrel. a glass/plastic/transparent cylinder will have ridiculous amount of refraction, that would distort the image of the BB. it would make it really hard to see where the inner walls of the barrel are. i think the best way to see if the BBs contact the barrel inner surface is to use a high-speed camera right in the BBs path. have the barrel/BB back lit so you only see the BBs silhouette. another option would be to have the camera side of the barrel machined into a concave shape to reduce the distortion from refraction.
Weapons Technician / Gunsmith
Don't look at me, I don't know, lol ¯\(°_o)/¯.