I’m betting the field mods were pretty freaky back then. The mounts must have needed constant review due to the beating after a few missions.
Working on the UH-1N’s at Howard AFB, Panama, 85-87 with Det 1, I do recall them speaking of NDI requirements for the tail booms after the 40mm’s ran their missions. Seems the constant thumping could really rack the bird, very noticeable by the crew when in flight, too. They said, “You don’t aim it, ya just point in the general direction and ride it.” Lol, woof, woof!
I wasn’t familiar with the XM123, but I did have a lot of time with the XM-93 system, GAU-2 miniguns, rkt pods, etc. The Hueys were escort for any nukes transiting the canal. I don’t recall the name of the 40mm system. I loaded it only once for certification, they never ran that system much at all in the mid-80s before I got there.
Funny, I’ve still have a busted flash suppressor, the old fork version, from the Gau-2 sitting in my model bunker with one tooth missing.