30mm Mk38 Mod4
The Navy has decided to replace the 25mm Mk38 gun mounts on Burke class guided missile destroyers with the 30mm Mk38 Mod4. They have completed their first installation. (Marines are also likely to upgrade to the 30mm.)
We have heard the Polar Security Cutter will get the 30mm Mk38 Mod4 but so far no indication other Coast Guard classes will get the weapon.
I would like to see all Coast Guard Cutters armed with the 30mm because unlike the 25, the 30mm can fire an air burst round that is effective against UAS. In addition the 30mm is also significantly more effective against small craft than the 25mm (“nine times the damage to the hull and five times the damage inside the hull”).
“The MK38 MOD4 Machine Gun System (MGS) is an upgrade to the MK38 MOD3 MGS to address deficiencies in meeting Counter Unmanned Aerial Systems (C-UAS) requirements. In addition, the upgrade will also improve performance against surface targets and maintain Anti-Terrorist / Force Protection capabilities. This system will be composed of the MK48 MOD2 Electro-Optical Sight System (EOSS), MK88 MOD4 Gun Mount (GM) and the MK134 MOD0 Console. This contract is to procure the MK88 MOD4 GM and the other components will be procured separately. The system is intended for installation aboard the U.S. Navy and U.S. Coast Guard vessels.”
The Mod4 is significantly different from the earlier Mod 2 and 3 in that the earlier units had the electro-optics mounted on the gun mount. In the case of Mod4, the electro-optics are mounted separately.
MK 48 MOD 2 electro-optical sight system (EOSS)
It might be assumed that any future installations will be the new 30mm, but replacing two of the earlier mods on over 70 DDGs with Mod4s will free up a lot of earlier Mod2 and Mod3 gun mounts. I am afraid the Coast Guard will continue to get 25mm gun mounts for their FRCs and OPCs unless we make a strong case for the 30mm.
Chuck Hill's CG Blog 
Chuck, you’ve been advocating for this for years. Finally, it is happening! I’m glad that the USN (and USCG) are finally realizing what the RN realized years ago, that the 30mm offers much more versatility than the 25mm, especially in today’s drone dominated environments.
Note, however, that unlike the onboard EO system of the Mod3, the Mod4 would most likely need 2 systems, one for each mount, particularly if they are located P/S.
With this introduction, I’m more confident that the new OPVs, the FF(X) and the LSM (Damen LST-100) will be armed with at least 2-30mm systems.