autogun said:

Possibly this is because BAE has been given a wide latitude to outfit the ships

Speculation confirmed - I've just been reading an article on the gun armament of the Type 31 in Warship World magazine:

"Type 31 acquisition strategy adopted a whole ship procurement model with minimum mandated equipment or government furnished assets/information. This meant the competing teams were largely free to make their own choices with regard to major platform systems, power and propulsion, and combat system equipments."

The article includes a detailed description of the ammunition for the 40mm and 57mm guns, and mentions a couple of guided 57mm rounds currently under development in the USA:

"The first of these is the High-Explosive, 4-Bolt Guided (HE-4G) projectile being developed by L3Harris Technologies under the Advanced Low-Cost Munitions Ordnance (ALaMO) programme (which is) intended to provide the navy with an improved smart round able to hit moving air and surface targets at significantly longer ranges.....the HE-4G round (designated MK 332 Mod 0) includes a radio frequency guided projectile.

The second development, known as MAD-FIRES (Multi-Azimuth Defense Fast Intercept Round Engagement System) is being led by DARPA. The concept is to combine the guidance, precision and accuracy of missiles with the speed, rapid-fire capability and large magazine capacity afforded by the 57mm. It is a rocket-assisted round with a terminal seeker to home onto an illuminated target."

Of course, there is no commitment at present to acquire these rounds by the RN, but it would be nice to have the option available.

Anyone know what the "4-Bolt" means?

autogun said:

Anyone know what the "4-Bolt" means?

It's apparently a reference to the course-correction method -- it has a set of four weights (bolts) that it can expel laterally to adjust the projectile's trajectory.

Edit: I finally found a video that shows this in decent detail.  https://www.youtube.com/watch?v=vY9rJBL1S2Y&feature=emb_title&ab_channel=L3Technologies

Thanks - interesting!

One thing I noticed from the video is that there was obviously an earlier version with six bolts rather than four.  That's how many objects you see being ejected in the slow-motion video and the live engagement footage.  At some point they decided that four were enough.

So if understand this correctly this system enables one correction in flight as it has to eject all the bolts not to leave the projectile unstable, while the sequence of ejection determines which way  it can correct.  

Wonder by how much it can alter the trajectory and is the trajectory change possible in every stage of the flight or just certain point as it seems the system of one nudge has to pick a specific point in flight to get the desired course change. 

Yes, it seems to be a single course correction -- it just times the ejections so they push the shell in the desired direction.  The seeker needs to be able to sense the target and have a pretty good measure of what the miss distance/direction is going to be.  The less the estimated miss distance, the later the correction can be.  Seems like it's probably fairly late in any case, judging by the test video.  That means the rounds need to be at least close to on target anyway. But against small boats, turning a near miss into a direct hit is a big improvement.  

The other interesting bit is that the seeker also serves as a delay fuze. The round doesn't necessarily rely on impact or a separate proximity sensor; it activates based on estimated time-to-go from the seeker.