There are general rules of thumb for figuring out which gun pack you need. They're not complicated, but following the above diagram is the best way to determine exactly, without a doubt, the size gun pack you require. What is important to remember is that the size delineations for the pack DO NOT correspond one-to-one with standard weapon sizes. In other words, the small pack is not exclusively for subcompacts, nor the medium for compacts, nor the large for full-size weapons. If an estimation has to be made, barrel length is the most optimal indicator of which size to select.
The small gun pack was designed to fit weapons with a barrel length no longer than 1.5 inches.
In order to fit into a medium, the barrel of the weapon cannot be over 3.5 inches in length for automatics, and 2.5 inches for revolvers.
Large packs can comfortably handle up to 5 inch barrels on automatics, and four inches on revolvers.
Extra Large packs are built for a maximum of 7 inch barrels on automatics, and up to six inches for revolvers.