It's a logical extension of AGS.  The concept is that the firing ship is too far offshore to be reliably targeted by shore-based weapon systems..  So, AGS - and ERGM and BTERM - was about 50 miles and railgun about 100 miles.  As the US Marines have pretty much abandoned amphibious attack, the need for long range gunfire support has evaporated.  And so, HVP is now repurposed as a cheaper air defense weapon which can also be used for land attack if desired.

gatnerd said:

The T15 57mm is really an awesome vehicle. 

.....

All in all it seems like one of the most versatile armored vehicles ever produced.

Remarkably similar in many ways to the  late 1970s Begleitpanzer 57 -- 57mm gun, coax MG, TOW or HOT ATGM, three dismounted infantry (scouts or ATGM team?) 

That's a much better photo of the Begleitpanzer 57 than any others I've seen. May I ask where it comes from?

It's hosted on Imgur.  I found it via Tanks-Encyclopedia, which credits topwar.ru.  But I'm sure it's a manufacturer photo originally -- it's also in the 1983-84 Jane's Armored Fighting Vehicles entry for the 57mm Support Tank, for example.

Worth noting that Tanks-Encyclopedia says there were just two human loaders in the troop compartment, not infantry.  Jane's says three infantry, but that might be a difference between the prototype and what they planned for a production version.

Thanks!

Some interesting look at the Naval Guns and options for the UK's new Type 31 Frigate:

https://uklandpower.com/2019/09/19/type-31e-light-frigate-weapons-options/

https://en.wikipedia.org/wiki/Type_31_frigate

Reportedly to have a mix of 57mm and 40mm guns, which seems a bit odd caliber mix.

Also 24 AA missiles, yet no obvious provision for Anti-Ship missiles. 

gatnerd said:

Reportedly to have a mix of 57mm and 40mm guns, which seems a bit odd caliber mix.

That may have something to do with the fact that Bofors, who make both the 40mm and 57mm guns, is owned by  BAE Systems...and guess who won the contract for supplying the frigate? 

Actually, I don't think that a formal decision about the armament has yet been made.  It would indeed seem odd if two new gun calibres were introduced for a handful of new ships.

I think you are right that no formal decisions have been made.  

Budget probably does exclude the Mk45 or Mk8 guns on the Type 31.  Which is fine -- callbacks to Falklands era gunlines are just not realistic and the only other role for 5-inch now is likely to fire things like the new Hypervelocity rounds for air and missile defense, which is clearly outside the T31's mission space.  The selection of 57mm Bofors or 76mm OTO probably should come down to cost, as both are functionally interchangeable.

On the secondary guns, the current service 30mm with the possibility of adding airburst munitions (AHEAD-style) and/or Martlet seems like the most versatile option while avoiding introducing a new system.  Plenty of small-boat killing capacity without too much cost or complexity. 

And although the chap at UKLandSystems complaining about how old Phalanx is, the newer versions are almost entirely different from  the 1980s version he is familiar with, with new radars, new electronics, better guns, and much better ammo.  A Phalanx Block 1B with Surface Mode would be a fine terminal defense.  Probably to be fitted for but not with in the current budgetary environment.

76mm has more range though. On another site, when I mention 57mm, several commenters go nuts. I then get a lecture on the superiority of the OTO 76mm.

Farmplinker said:

76mm has more range though.

Not nearly as much more as some people think.  A lot of reference books are doing apples to oranges comparisons, things like maximum range versus maximum effective range (very different) or HE versus sabot rounds.  Best I can figure, the real-world difference is about 10% in favor of 76mm.  That's not much.