My apologies for the delayed post. I was trying to reverse my steps. I pulled up the primal info on both base files for machinedpistol and advanced ammo which I used as a base, compared both them side-by-side (really nice that it allows you to do that), and fixed a few things here and there to make them identical and did the same with machinedpistol against my mod. corrected a few things… then, looked at the weapon info in the area you suggested, and got a bit confused. Like everyone else, I have four possibilities.
Support drag onto item class:
Support drag onto item classes:
Ammo support drag onto weapon item weapon template:
Ammo support drag onto weapon item weapon templates:
I’m assuming only the last two fields are relevant. Neither actually shows my mod’ed ammo in the list.
…one step closer…one…step…closer… 
Edit: deleted all the annoying references to the old weapon mod since they kept ‘re-appearing’ after each load of the editor. lost the ammo and weapons files I was working on, but, the folder they were in no longer appears.