In the interest of saving a big chunk of money, I think the Coast Guard and Navy should reconsider the proposed fitting of the 57 mm gun to the new Offshore Patrol Cutter (OPC), and perhaps recycle existing Oto Melara 76mm Mk 75s instead.
If the Navy is going the stop supporting the Mk 75, we probably have no choice, but the 76mm is one of the most widely used weapons in the world. New developments including guided munitions and high velocity discarding sabot rounds are keeping it competitive.
The two systems are very similar in weight and range, with the 76mm having a slight edge in range. The 57mm has a higher rate of fire, but this is balanced by the greater weight of the 76mm projectile.
It may not save the CG a lot in acquisition because, if it is like past programs, the Navy is buying the gun, but the Country has to pay for it, and the CG might benefit in having to retrain fewer people to maintain the weapons.
I think the choice of which is best boils down to what you think the critical threat we need to address is. If it is airplanes and cruise missiles or huge swarms of Iranian speedboats, the 57 mm might be better, but if you think you might have to sink a ship or hit targets ashore, then the 76 is probably better. Frankly I think the latter is more likely. For that reason I would actually prefer we mounted a 5″ Mk 45, but I don’t think that is going to happen.