I'm on colonel rank now and the guns (on shop NPC) are organized quite confusingly. Not by type, not by name, not by craftability, also not exactly by tier (higher/newer tier is usually on the lowest part but not always).

The best organization I can think of is like:

handgun (tier 1)
handgun (tier 2)
handgun (tier 3)
magnum
magnum
magnum
sub machine gun
sub machine gun
sub machine gun
rifle
rifle
rifle
sniper
sniper
sniper
machine gun
machine gun
machine gun

Should be easy to browse around and check what's new on each type.