To fit in the shrouds, the barrels must meet some requirements:
US Army Alpha Black & Bravo One - 9 inch Minimum Length x 1 inch Maximum Diameter
US Army Project Salvo & Sierra One - 9 inch Minimum Length x 1 inch Maximum Diameter
US Army Carver One & Tango One - 4.5 inch Minimum Length x 1 inch Maximum Diameter
Most barrel diameters are less than 1 inch. To compensate for smaller barrels that fit inside the shroud, you may need to add a shim for a tight fit. This can easily be done with a few wraps of electrical tape (a paintball players best friend). Add where the front of the shroud is going to meet the barrel when it is fully installed.