Well and the actual lower receiver being milled to accept the different FCG.
But to answer your question, they have a mil-spec buffer tube. The parts that are not mil spec have to be that way under BATF regulations. They can not be easily and readily convertible. ALL of the parts that can be mil spec, are. It is as much mil spec as any other on the market.
" The strongest reason for the people to retain the right to
keep and bear arms is, as a last resort, to protect
themselves against tyranny in Government...." - Thomas Jefferson