My reply did not fully cover the question, and I apologize. A licensed manufacturer of NFA firearms may build a machinegun for "demonstration" purposes and for potential sale to LE or the military and keep it in stock for "trials". It cannot be sold to an individual, but BATFE insists that an FFL of any kind is a business license, so a licensed manufacturer who makes guns for his personal use might be deemed to be not doing business and his license could be revoked. If that happens, he might be allowed to transfer any guns he made to another SOT or to an authorized agency. Otherwise, they would be seized. As you gather, a lot of ad hoc judgements are involved in that sort of thing.