You've got a couple of choices. You could get an item editor from this site and edit item 359 (in data\proto\items). Cubik's item editor works great.
The other option is to delete the file .0000359.pro from the aforementioned folder, so the default file from the master.dat will be used.
However, both of these carry some risk of problems. All of the maps with existing caseless ammo will have certain values with regards to weight, number, etc. Maps where the ammo is generated on the spot (such as in random encounters) will have the caseless ammo with the stats after the modifications you have made.
You could use the mapper program to edit the maps that have caseless ammo already there. Or, you could just change the item, and deal with any potential bugs. Not quite sure what that entails. It might just be that the ammo will not stack properly when you get it (i.e. you have 25 rounds, pick up a stack of 50, and you have 125, for example). Or, maybe a file that Nirran created (already in the MM you have) will fix the issue. Or, maybe there will be some other unforeseen problem. I can't tell you with complete certainty.