The gunsmith will set the barrel and barrel extension up properly for the correct headspace with the provided bolt, then drill the gas port.
If you by your own blank, barrel extension, and bolt, you can keep costs down. As long as a reputable smith does the work, it will be good-to-go.
Jeff at GA Precision knows how to build reliable, highly-accurate auto-loaders, and cutting the chamber with a reamer that is more appropriate for an auto-loader is half the battle. Many production guns have very tight chambers that work fine with factory ammo, but even full-length re-sized cases don't run well through them.
I've had that happen with the DPMS LR-308. With the .308 Jeff built me, it eats everything it's fed and shoots .5 MOA for 5 rounds with the 155gr Lapua Scenars.
You can also ask for a chamber to be cut for a specific bullet that you plan to shoot if you're looking for better accuracy.
Also, even using the DPMS receivers, I would get a Tactical Machining or other billet lower with better ergonomics than a DPMS, and more importantly, I would suggest getting an Armalite AR10 barrel nut and Armalite bolt carrier group, since they use a spring-loaded firing pin and balanced lug arrangement on their bolts.