For a FIRM 600, a person can build an AR-15 that's got every "must have" "fighting carbine" feature (as determined by internet commandos on BARFCOM), if one can get a 10 dollar FFL transfer and are willing to wait for a Surplus Arm and Ammo lower to come in stock at AIMSurplus, or for another lower (like Palmetto State) to go on sale for 60 bucks or less, and are willing to settle for the crappiest rail mounted UTG brand rear sight, and basic furniture.
Or a person can choose to forgo chrome lining (which will increase your preventative maintenance requirements, shorten your barrel life, and increase your accuracy), saving 45 bucks, for a nicer sight, stock, or handguard set (or a more expensive lower), or a person can forego M4 feed ramps and choose an A2 upper (with fixed carry handle).
A person can do this by purchasing a rifle kit from
Del Ton.
One my ARs is a Del Ton kit rifle, and while I'd probably option it different (it's a 20 inch HBAR, and weighs a million pounds), if I were to buy it all over again, I still really like it, and it's been an excellent rifle.
I know that Del Ton didn't always follow all of the QC steps listed in the chart, but that's changed in the last year (though, frankly, my rifle, which is pre-new-QC-steps, is made of better components than the RRA and CMMG rifles I've seen, used, and owned).