I crossed over my 200th ground up build this winter, worked on hundreds of other AR's over the years, and the last time I checked my batch-tracking sheet, I was over half a million reloaded rounds to feed them (vast majority = 27.3grn Varget under 50grn V-max or 55grn HPBT). They tend to beat the he11 out of brass, especially the way the factory AR industry has gone with a pattern of severely over-gassing their actions to ensure reliability.
To steal the term from Kimber; doing the "melt job" on the barrel extension lugs takes elbow grease and time. It's easier before you install the extension, but most folks aren't equipped to remove or install one on their own. I have a few sets of mini-files just for this purpose, including some with the handle/spindle cut off so I can PULL the file from the head end, to let me work the backside edge of the extension lugs from both directions. The Popsicle sticks, paper, and compound work well to polish up once the major cut is made - and again, the same melt job should be done on the bolt locking lugs as well. Mapping the bolt to the extension once the rest is done ties it all together and helps reduce the risk of broken locking lugs (and potentially improve accuracy).
It's not rocket science. The AR-15 was designed to be serviced by grunts with an enlistment IQ requirement of 80 (benchmark average person is 100). They aren't magic. Just spend the time to fix the laziness built into your rifle by factory producers. Gouges on your NECKS which are that deep can also be driven by an over zealous ejector spring or an over-gassed action (way too much force going on at that part of the stroke). Smoothing it up will help, but might just leave you with dented necks instead of gouges.
Any CHS or rim damage? How much is your shoulder moving forward upon firing? Any idea what your throat dia is? What's your loaded round neck dia vs. your fired cases? Drop the tip of your caliper into the gouge - how deep is it really?