The Vickers 3-pdr guns which saw service with the RN in WW1 were more powerful than the usual Hotchkiss 3-pdr naval, using a larger cartridge case to achieve a considerably higher muzzle velocity (both guns apparently fired the same range of projectiles). The guns and ammo are described here: http://www.navweaps.com/Weapons/WNBR_3pounder_V_mk1.htm
I have just acquired a cartridge case for the Vickers (I also have one for the Hotchkiss), and am a bit curious about it.
The dimensions are:
Vickers: length 413 mm, rim diameter 68 mm, chamber volume 64 cu/in. Barrel 50 calibres.
Hotchkiss: length 376 mm, rim diameter 64 mm, chamber volume 43 cu/in. Barrel 40 calibres.
According to Friedman's Naval Weapons of WW1, two different Vickers 3-pdr were made: the Mk I and the MK II, both using the same ammo. The Mk I saw a lot of use, later on in high-angle mountings as an AA gun. The Mk II (which he also describes as the "Vickers Mk C") saw very little use, only on board the Severn class monitors.
Both Mk I and Mk II had the same barrel length and the same chamber pressure, so the performance should have been the same, but he quotes the Mk I as achieving 2,587 fps and the Mk II 2,680 fps, despite the fact that he quotes a slightly lighter shell for the Mk I (for comparison with these, the Hotchkiss achieved 1,867-1,927 fps).
My case is stamped VICKERS II. I am puzzled as to why the gun type was marked on the case in this way, when both guns were supposed to be using the same ammo.
Does anyone have any explanation for the differences between the Mk I and Mk II Vickers which might affect the ammo and its performance? Also, are the Mk I and II cases equally uncommon, or is there a difference?