Re: US Military Load developments
Just a guess, units allowed discretion in such things put out a request, which goes up thru command, where the lawyers look it over and decide if it meets the requirements of the rules of land warfare and whether those rules or applicable, with that blessing if command approves then they no doubt have a contractor or probably less likely their in service marksmanship unit try to make it happen, possibly with potential end users involved in testing the round and providing feedback. Think 6.8SPC.
The occasional ambitious individual may on their own come up with a proposal and I suppose its possible create a sample on their own time, I doubt it would ever be allowed officially to be fired thru a service weapon though. But there are not backrooms in the team house where they crank out ammo for testing.