This is fun.
5.56mm
Take a page out of the Saddam & Nork play book. Say you will build more. Show scientists working, tin cans and missile shells and then save the $ and don’t build them, who would know? They are secret. Tell everybody you got a lot of them now. Let everybody else burn GDP to catch up ;)
Not like you need a lot of them, nor can you really test them either.