25mm is acceptable and should really be the minimum fit. If you need anything bigger than that, we have other platforms available that are likely to be more suitable. 25mm should easily out range anything pirates or smugglers are using in/near our waters.
Sabot rounds will do about 1500m/s and can easily engage targets at 1-2km. Ak rounds would be like air-rifle/.22lr energy/velocity levels, and wildly inaccurate. Probably don't have crew standing around outside, but at that range with a AK they would be lucky to hit an aircraft carrier/the ocean at that range. Most aks are notorious hitting anything even at 100m range, but the projectiles would be lethal at 100m, but at 2km its like a falling projectile rather than a fired one. Meanwhile, the 25mm would have 1000 times the energy at that range of 2km.
.50 cal would still easily outrange a AK, but put crew and everyone at a higher amount of potential risk. More of a 1km option. But for civilian ships having a high powered rifle. The USN ended up using a high powered rifle to deal with pirates off Somalia. Its always nice to have, options.
With ships, moving a little further back and watching is always an option, particularly if the ship isn't going anywhere and waters aren't crowded.
If you were desperate, there are still a few 40/60s in stock;
I thought these were only decorative and used only to avoid paying port fees.