Put some WD40 or heavier oil down the barrel. It provides an incredible seal between the pellet/bb, the lubricant provides excellent lubrication and sufficient film strength. This can add a lot to muzzle velocity. I used to shoot through the side of steel pails at 50ft  with my SW .177 air pistol using steel BBs when I was a kid. I could only put a good dent in them until I used this oil trick.