Military Rounds are a uniqie kind of ammo in that they function as both actual ammo AND as currency.
With that in mind you must have 1000 rounds AT THE SAME TIME to get this trophy and while this ammo isn't totally scarce you must be careful on spending.
The last place you can spend the rounds as currency is in Polis (in the final third of the game) and once you leave that area you can't return later. There's another shop in the church but you can't make sales there, only trade or throw away inventory.
When I made it to Polis on my first run I had 7 or 800-something rounds and was planning to just sell my other ammo, inventory, and guns for the difference; I was shy 42 bullets to reach the 1000 so I planned to do it on my next run and splurged on ammo, gun mods, etc to get fully geared up, putting me in the low 500s.
At the end of the D6 chapter I had over 1000 bullets, most of which were found in that very chapter.
Lockboxes are your best friend for an ammo boost and if you search every toolbox or steelbox you find in D6 you should be fine but don't spend carelessly just in case you overspend that one bullet