I would say 16" for a naval gun but it is not a self contained cartridge.

Next would be 357 magnum but it is limited to revolvers. (Yes I know about the various 357 mag autos. A Desert Eagle is not exactly a belt gun for everyday carry.)

SO, my vote goes to the 10mm. Load it down to a light 40 S&W level or up to near 41 magnum. You can find it in a revolver, auto-loading pistol or a carbine.