No, it's not snake oil. Like Pappy said, they are just ensuring the bolt properly headspaces on that extension. Bolts are fairly inexpensive, so it's not a bad idea to change it with each barrel, especially high-dollar barrels that typically will go 5000+ rds before they need a change.
Headspace on an AR is the interplay between the bolt and barrel extension. If those two parts are within industry spec. then headspace is good. There's really no way to tweak it to get better headspace for accuracy. The parts are either in spec or not. If headspace is off, you have bad parts...
The bolt is the wear item that needs periodic replacing. Not often, but every couple of barrels or so. The carrier doesn't really see any wear at all.
I just drive over to Windham Weaponry whenever I get a sale notice that they have complete BCG's for $100. Nothing special; just mill spec parts.
I wasnt disappointed by The Pacific at all. Maybe too much time wasted on love stories in Australia and back here in the U.S., but top notch other than that.
Read the books on which it is based: With The Old Breed on Peleilu and Okinawa and Helmet For My Pillow.
Maine deer are few and far between. You can do everything right and get skunked while the guy who does everything technically wrong tags out every year.
Never discount the "shit luck" element when it comes to deer hunting.
Maybe in a shop setting where you are in the same spot every day. Other than that, i see no need to ever again use the corded tools I have.
If you don't think battery tools have the power of corded, you haven't used the new generation. The convenience of not having to plug in far outweighs any...