Is there any reason to buy one of these from a store instead of buying aftermarket parts and building your own? Is there any part that you cannot buy? I'm looking at some sites that sell parts and it seems after you change the barrel, stock, trigger, etc. then there is nothing stock left. I realize it costs a lot to build it but seems cheaper than ading the cost of a stock one just to change all of the parts anyway. Is there something I'm missing?