Originally Posted by MacCool

Test firing the assembed weapon for function is commonly done, but pressure testing isn't necessarily. The M4 TDP calls for HPT and MPI of the barrel and bolt. Those individual components are individually tested with a high pressure proof load, then the whole rifle is test fired for function.

HPT and MPI are a little controversial. Not sure that it's necessary given modern manufacturing techinques and materials, but it is true that because of the expense, it's one way to distinguish the higher-end weapons