I put one together with a Radical Firearms upper and a self assembled lower from a NES group buy. Watch for sales from Radical- sometimes it will price out cheaper than you can build and the components are nice quality. It's a ton of fun- almost literally as it generates nearly 2,200 ft-lbs of energy at the muzzle. Other than some FTE's when trying to use a brass catcher (I'll explain why later), this upper has been flawless. The comp looks a little funny but works extremely well. A nice feature of the Radical upper is that the ejection port has been given extra clearance for this round. It's not mandatory but a good idea.
FPS Russia states different mags necessary but that's wrong- if you live in Mass you can use your preban 30 rnd mags to hold 10 rounds of .458 socom, single stacked. They make centered single stack followers but I just use my standard AR mags.
This round is FUN to shoot! Last range trip I brought a buddy who's full time guard and gets to shoot 5.56 all the time courtesy of Uncle Sam. We had a great time busting up range allowed stuff and punching holes in paper at 50 yards. We were a little surprised to see that after multiple rounds at the same bullseye, we had blown through nearly 3 feet of berm sand and almost blew through the rear support timbers.
The bad news is yes, expensive to shoot! Probably ~$2 per round for ammo depending on what you can find. Pretty easy to reload, however. Starline brass will set you back ~$.76 per. Not sure how many reloads i'll get but seems like my loads are not working the brass too hard and I hope to get ~5X. Projectiles are not cheap either- 300 gr HP's feed great in mine, about $.23 to $.28 per round depending on source. Add primers and powder, your probably looking at $.65 each at best assuming very good life from the brass. I had 4198 which is a good one for this cal, so I have stuck with it. Maybe with some work could do better on the cost. That's why I tried the brass catcher, and at a given NES shoot you will see me crawling around looking for that last piece of brass, LOL.
I am traveling overseas this week, so when I can get around to it I'll post pics if I have access to any. IMO, it is a pretty badass round and is virtually identical in energy to typical .45-70 loads, at least in the 'lighter' projectile weights, like 300 gr and 325 gr.