Tony Williams lays out the history and options for so-called Medium Caliber Guns (MCG) for warships.
The two primary segments of MCGs in use in the West today are the "light", rapid fire guns (57mm Bofors, 76mm OTO-Melera), and "heavy", slower-firing 127mm guns (Mk45, 127mm OTO-Melera).
IMHO, both the "light" guns and "heavy" guns are problematic as generic, "Swiss Army knife" naval mounts for smaller combatants.
"Light" guns typically have high ROF, but small munitions. Guided 57mm and 76mm rounds are in development, which should improve anti-swarm and point defense capabilities, but NGFS is still marginal. The 76mm HE-POM round is somewhat less capable than an 81mm mortar round.
The public rationale for removal of the 57mm Mk110 from the DDG-1000s in favor of 30mm Mk46 mounts was due to the Mk110's less than impressive performance in trials (presumably vs small craft).
There has been some debate about the effectiveness of 57mm or even 76mm rounds against larger vessels.
So are these "light" guns really gaining us much that one or more autocannons couldn't handle?
"Heavy" guns have much lower ROF, but larger munitions. Long-range NGFS munitions are in development, but their AAA/Point Defense capabilies are next to worthless at this point. Heavy guns also aren't suitable for smaller combatants.
My back-of-napkin roles/requirements for an ICG,
- Suitable for smaller combatants (down to 500-1000 tons).
- Useful NGFS capability. It doesn't have to be a STOM/OMFTS wunder-weapon (e.g. AGS, rail guns). It just has to be a valuable participant. Think Five-Inch Friday, Operation Market Time/Sea Dragon/Sealords, and NGFS during the Falklands conflict.
- Useful AAA/Point Defense capability with guided munitions (e.g. Strales/DART).
- Anti-FAC/FIAC swarm capability.
- Greater ability to sink or disable larger vessels than "light" guns.
- Scalable response
- Signalling (e.g. firing across the bow)
IMHO, the various 100mm guns (French Compact, and the Russian and Chinese models) are the closest to my ICG requirements. The French Compact 100mm gun has a ROF up to 90 rpm (burst) and weighs around 14 tons (mount only). This is a bit more than half the weight of a Mk45 mount, and less than half of a 127mm OTO-Melera mount. However the caliber and design never really caught on. The Russian's put the similar 100mm A-190 gun on ships as small as the 500 ton Project 21630 Corvette.
How about design a similar gun mount around either the semi-fixed 105mm howitzer round or the 105mm M68/L7 tank gun round? Both are shorter (but somewhat thicker) than the Compact 100mm round. Or better yet, design a larger case (and gun) for the 105mm howitzer projectile family that can approach the performance of the Denel G7/LEO 105mm gun.
All the work on various new and existing 105mm projectiles, fuzes and guidance options could be adapted for naval use.
A high ROF, 105mm naval mount with G7-equivalent performance could provide NGFS up to 30km away with just base bleed projectiles, or 36km with VLAP projectiles. Greater ranges could be obtained from Vulcano-style or rocket-assisted rounds.
With a sustained ROF of 45rpm, a single mount could deliver a 105mm gun battery-equivalent fires. Three mounts could deliver battalion-equivalent fires.
I know there isn't much stomach for developing naval gun mounts these days, unless they are small autocannons, or giant wunder-weapons. However a relatively light mount, using existing projectiles and/or complete rounds, with a high ROF, would appear to fill a useful niche.