Thought this video was interesting enough to pass along.
This makes it clear why the nominal “220 round per minute” rate of fire for the 57mm Mk110 is only good for short periods. There can be up to 120 rounds on the mount but there may be interruptions to reload after 20 or fewer rounds. Frequently different types of ammunition are loaded in the left and right sides of the mount ammunition feed system, so there may be only 60 or fewer rounds of a particular ammunition type on the mount.
The linked information above includes a note for the 57mm Bofors 57 Mk2 which appears to share the same reload mechanism as the Bofors 57mm Mk3 (Mk 110 in US service) “Time to reload 120 rounds: 2 minutes”. It appears reloading the two 20 round ready service cassettes that move with the gun requires the gun to at or near maximum elevation. It is not clear if partially filled ready service cassettes can be topped off, I would hope that would be the case. The time to elevate, reload and return to firing position could be critical. Hopefully you will not wait until all 120 rounds on the mount are expended before reloading.
Thanks to Lee for bringing this to my attention.
Chuck Hill's CG Blog 
Don’t forget, it’s an unmanned gunhouse, so you’ve got to send someone out there, grab ammo from the ready service locker, or the magazine, and schlep it to the gun, and then reload it, and return to a safe position. That’s all going to add a little more time.
Hopefully, targets will require somewhat less than 60 rounds each!
In the case of both the 57mm and most 76mm mounts, the reloading of the on-mount rounds is done from below decks. On the 76mm Mk75 gun still on 270s, the ready service rounds are on a carrousel below the mount and can be reloaded without bringing the gun to a predetermined reload position. On this mount, https://chuckhillscgblog.net/2022/07/17/new-76mm-gun-mount-solves-frequent-siting-problems/, a reload from below the deck is optional.
I would hope most individual air targets will require less than 60 rounds, but there is also the possibility/probability of multiple inbounds.
Against a large surface target, even 60 ALaMO rounds, may not be sufficient for a mobility kill.
When comparing, keep in mind the 76 mm rounds are about twice as large as the 57mm rounds so they are likely to individually more effective.
With the right rounds it would have made (?) a great CIWS.
We apparently already have a guided round for this gun in the form of the ALaMO but that was not expected to work against high speed targets. https://chuckhillscgblog.net/2021/08/25/remember-the-alamo/
There is supposed to be a guided round capable of hitting inbound cruise missiles, Multi- Azimuth Defense Fast Intercept Round Engagement System (MAD-FIRES), in development, but I have not heard anything about this program since award of a 27 month development program that was supposed to have been completed Nov. 2022. https://defbrief.com/2020/09/01/darpa-contracts-raytheon-for-third-phase-of-mad-fires-program-development/
Does look like the 76mm might have been a better bet. They already have a guided round with a capability against cruise missiles and I don’t think it has the time outs for reloading that the 57mm does.
With the way the Mk.110 was designed and appears to perform (at least with Alamo rounds), it is ideal for anti-small boat attacks. I would hope 60 rds of Alamo would be good for at least 10 small attack boats like the Iranians use. All 120 rds being Alamo should easily terminate 12-15.
It seems to me a better design would have been to feed the hoists into the bottom of the intermediate magazines, so they could be reloaded after moving their clips of ammo into the cassettes. If they had done that there would be no point in time when the mount had to be empty…
It seems to me if the hoists were built to feed directly into the Intermediate Magazines, they could be refilled while the Cassette is off refilling the primary magazines. This way, the mount would never be empty and always have 40 rds available to load…
I love reading peoples comments when they have minimal knowledge of how this gun ACTUALLY works. Ours feed into the bottom of the gun using the installed hoist from below decks. The rounds are programmable. Combat load is a single type of rounds.