If you want a second copy, you're going to need to perform the Project again and hope the RNG smiles on you. You do not unlock the ability to directly purchase additional copies of a given Ammo type or anything like that.
Note that this is a single copy of the Ammo type, and nothing more. In short, performing Experimental Ammo spits out a random Ammo type from among the 5 types I'll be covering in this post, each time it's performed. Experimental Ammo is a Proving Ground Project available the instant you've built the Proving Ground, and it's one of a handful of Projects to share a unique gimmick: randomized results.
