I was on CHA during OP MOBILE - I worked with our SWC on a bunch of info for NGFS. It was not anything we employed, but we put a lot of effort into it.
The 57mm on the Halifax Class has a rate of fire of about 220 rounds a minute, with 120 rounds on the mount. It can fire 40 round bursts without needing to engage in a 'reload cycle'. Firing a full turret load of 120 rounds (not including 1 on the shift-tongue) would take about 11 seconds per burst of 40 rounds, then a 10 second reload, then 11 to fire 40, 10 to reload, then another 11 seconds.
Normally you have 2 types of ammo in the turret - some designed for AA, some for surface targets.
If you used only one type, you could dump 120 rounds into a target area in 53 seconds.
Each 57mm shell has about 400 grams of HE in it, and is about 2" in diameter. You'd be, effectively, firing 120 60mm mortar bombs into a target area in less than a minute.
That seems to be a useful capability. Alas, it was not implemented during Op Mobile.
Interestingly our SWC ended up on a certain destroyer, and may have brought home a duffel bag with a 4.5 inch shell in it...that may have been fired while he was onboard that destroyer.
One of the things I put together in the year following that deployment was an article for the Marine Engineering Journal. It ended up getting published in 2012. Summary - strip out the torps from the Stbd Torp mag, clear the Stbd Mezzanine deck, install a 1.9m turret ring, put in a 120mm Naval AMOS/NEMO turret, and you'd have the ability to deliver supporting fires, including precision guided mortar rounds with very little loss of capability. (Retain Stbd Tubes, but store reloads only in the Port Torp mag.)
Biggest problem was range - the limited range would put you VERY closer to shore, as compared to having a 127mm gun up forward. However. There is no good place on a Frigate to install a 127, the entire structure would have to be re-done wherever it was put.
Here's a link to the article I wrote:
Begins on Page 3.
NS