Sorry to hear of your frustrations on your .458 SOCOM build. I purchased a RRA .458-LAR complete rifle in 2007. It has run like a Swiss watch right out of the box. I have only used my own reloads as ammunition, and have been very pleased with the results. Like you, I only use milsurp 20 and 30 round M16 magazines. Soon after buying the rifle, I bought a cartridge gauge from Marty Ter Weem, the man who developed this round. This has been a great help in reloading. However they are hard to come by at this time. Since Marty is currently working overseas, he contracted with SBR to build the parts (including these gauges) for the .458S, but I keep reading they are behind in keeping up with demand. (By the way, Marty is the “some guy from Texas at Tromix” that you reference in one of your posts.)
Again, I read that Marty has only contracted with RRA and SBR to build .458 parts and firearms. Tromix has stopped their production on this rifle some time back. Only these companies can or could chamber barrels using Marty’s chamber reamers. So unless the barrel you have is made by RRA, SBR or Tromix, it will probably have been chambered with a reverse engineered reamer.
You state in your original post that “It has never worked correctly. I have found out that this is not unusual. A lot of unhappy 458 SOCOM owners out there.” I have not seen this over the past few years. I hang out here on the AR Variants page as well as the Indiana Forum. (Back in 2006 the .458 SOCOM became one of a limited number of center rifle cartridges that were made legal for deer hunting. Prior to this we could only use shotgun slugs, muzzle loading rifles, and certain centerfire handguns.) I also cruise .458socomforums.com. I see occasional issues with rifles in this caliber, but not a great number. 2 or 3 years back, RRA seemingly used chamber reamers past their useful life and some guns and upper assemblies were shipped with rough chambers. These were fixed upon return. Corbon ammo released a lot of “hot” ammo that sheared locking lugs off some bolts. RRA still has a warning on Corbon ammo posted on their website. And, as you say, all factory ammo has been problematic from time to time. I am sure there were other minor issues. But for the most part, the guns have run well both for hunters (for deer, hogs, and larger game) and operators in harm’s way for whom the caliber and uppers were originally developed.
Hopefully your handloaded rounds will correct your failure to feed situation. If it does, I know you will be happy with your rifle. You might take a look at the threads over on the .458socomforums.com. There is a lot of useful info there.