It's easier to mod for a couple reasons.
1) If someone added this and already researched the base game ammo then it wouldn't work without a script as part of the mod, not the hardest thing to setup but at the time I made this a few years ago I wasn't comfortable messing with that.
2) If another mod also toyed with the base game research in some way then this mod would either not work or disable the other mod. Then you'd have to be aware of exceptions for any mods out there that come up and put THAT into your mod also.
So much easier to just make it a research, it then doesn't interfere with anything and if you remove it, the stuff just vanishes and no harm, no foul.
I also do not require you even research the other ammo, just it has the same pre-reqs AS the other ammo. So if you were a shotgun person you could get this and not the other, or get it first and go right to your ammo of choice.