Just ran across this process outlined here:

http://firearmsdesigner.com/?p=153

I thought this was a great reference with plenty of pictures and a good explanation. It took the mystery out of it for me anyway. Anyone decent with a lathe could handle this just as easily as barreling a M98 or a Remington 700.

Also interesting was the topic on Modifying an AR-15 bolt for .45 ACP because the 458 SOCOM has the same .473 rim diameter as the 45 ACP. From what I have researched, factory 458 SOCOM bolts begin life as standard 5.56 AR bolts and are simply opened up to accept this size case. Strength is still adequate because chamber pressure is kept low enough.

And so, the topic here of Building an AR-15 in .458 SOCOM is pure gold. Using the bolt head modification, a standard barrel blank in .458 bore, and a normal barrel extension modified for center feed, this is not an outrageous project. The work to enlarge the ejection port is not hard, and you could do it without a mill, if it came to that. It's light , nonprecision cuts and a milling attachment for the lathe would even work.

I just thought this was great info for anyone looking at gunsmithing on AR type rifles. One could do a build in any custom caliber with some of this knowledge. The only hangup would be making sure magazines worked right.