Hey whats up .
I'm no expert but I'll give this a try but Im going to stop typing in 10 min .
Lets start with mil-spec . This stands for military spesifications and generally speaking thats a good thing .
Things you would like to be mil-spec .
upper and lower receiver ,
BCG (bolt carrier group ) The gas key is part of the BCG .
I did not put barrel here cus a mil-spec barrel has a specific contour ( shape ) twist rate and are chrome lined . the military has very specific reasons for these thing and they are not really necessary for civilian use .
Forged - Is a good thing and a must for your receivers IMO
Magnetic Particle Inspected ( MPI ) is a nondestructive test to determine the existence and extent of macrodefects such as cracks in ferromagnetic materials; discontinuities in the material create variations of magnetic field which are outlined by fine magnetic particles. Also known as magnetic crack detection.
What do you want MPI
Chrome lined - Chrome Lining is a process where chrome is 'welded' to the steel . This produces a coating that is twice as hard as the steel itself. It prevents corrosion, makes cleaning much easier, and extends the life of the metal .
Chrome lining is almost always a good thing . Some people do not want the bore chrome lined cus it reduces accuracy