At least half of the AR15's being used today do not meet the mil-spec, they have 4140 barrels, 8620 bolts and commercial size RE's, but they still run fine.
Bolt= Carpenter 158
Barrels= Depends on model, but they do use 4140, 4150, and 416 SS
Receiver Ext.= Depends on model. (My MK 12 came with mil-spec receiver ext. and SOPMOD stock.