Newb here. I know very little about 458 SOCOM.
What's a good way to get started? I have a spare AR15 lower, should I get an upper, or would it be better to get a complete rifle?
Can I get ammo, or would it be better to reload my own?

Any help is appreciated.

You can buy just the upper. It was designed to run on a standard lower without modification

Buy Tromix. Don't look elsewhere.

You can definitely save money hand loading. I reload all center-fire calibers I shoot. The 458S isn't particularly difficult; you just need to be sure to keep pressures under control as the pressure on 458S is significantly lower than typical AR calibers due to the large case head and weaker bolt lugs.

Thanks Obessed. My next question was going to be about Tromix.

I'm looking at a Tromix upper with their 4 port brake, lightweight barrel and heavy duty billet receiver.

also if you are looking for one, look for brass too, Starline only makes it certain parts of the year but there is usually some for sale on GB

TROMIX or you will be sorry.  Their LW barrel with 4 port slim line brake is one of my favorite setups.  If you want to build your own upper, you can use any upper you want but you will need the ejection port opened up on most of them. Paladin Machine Shop Service (tp555(at)windstream.net) can do that for you for very little money and at the same time lathe true the front of the receiver.  You'll spend less than $50 and have it back in a week, two absolute tops if he is really busy.

 Any bolt carrier that suits your fancy will work but get the bolt from TROMIX.  I usually buy a black nitride BCG from AIMS  (overpriced right now at $119 but at least they are in stock) then swap out the bolt with a TROMIX bolt.  A TROMIX bolt is going to be made right with no short cuts.  It will have the proper extractor, ejector, and bolt face diameter.  Others cut corners by half-assing the extractor and ejector and often one runs into trouble with these incorrectly modified .458 SOCOM bolts. Their barrel will have the correct chamber and the barrel extension will be correct as will the gas port.  If you want trouble free shooting from day one, go TROMIX or rue the day you didn't.  

This is the first time I have seen Starline brass back ordered in a couple of years.  They normally have it in stock and I have not seen it OOS in ages so things must be mucked up due to the hording going on.  It probably won't be too long though before it is back in stock.  Maker Bullets normally has it for .68 each but they too are OOS right now.  If you find some, get it, but don't panic and over pay for it.  I certainly would not pay more than .75 a case for it because it will not be OOS forever.  

If you don't reload, this is a good reason to start.  Midway has quite a bit of .458 SOCOM ammo in stock but it's $3 a pop.  However, you will get ammo to shoot and brass to reload so take that into account.  There is really only one maker of .458 SOCOM brass and that is Starline.  They make it with whatever head stamp you want so regardless of what the head stamp might say, it's Starline brass so you can mix SBR and Lehigh and Starline brass with impunity.

By the way, if you see a pierced primer, DO NOT PANIC.  NOTHING IS WRONG.  It is very common for the rifle length firing pin to pierce the primer once in a while of the large PISTOL primers used in the .458 SOCOM.  Just take out the firing pin, polish the tip and make the firing pin protrusion no more than .030" from the bolt face and no less than .028" and the piercing will stop.  Standard firing pin protrusion mil-spec is .028 to .036" and that is a bit long for the softer pistol primers and they often pierce.  Just polish the tip back with 600 grit wet/dry and buff with Crocus cloth or a Dremel felt wheel with polishing compound and you'll be good to go.  THIS IS NOT A DANGEROUS CONDITION!  Do not adjust firing pin protrusion until you see a pierced primer because it may not be needed.  Don't fix anything until it needs fixing.

Mags, use most any QUALITY AR magazine, however, the P-Mags do not play well because of the front ridge down the middle of the mag.  They work for shorter FN bullets but spitzers often cause feeding problems from those mags.  I currently use clear Lancer AWM mags exclusively in my .458 SOCOMs because I like seeing the cartridges in the magazine, but most any quality magazine will work.   A half moon cut in the front of the mag is not needed for shooting (unless shooting very wide meplat bullets) but the cut makes unloading unfired rounds from the magazine easier, so I use a Dremel 3/8 sanding drum to do the half moon cut.  It takes all of 30 seconds to do in the Lancer polymer magazine.  And it will not affect function at all when used with 5.56 ammo.  You do not need a special follower unless you are using for hunting in a state that requires it.

On your lower, it was designed to use standard weight buffer and buffer spring, both in CAR configuration or rifle configuration.  Don't start off with HBs and XP buffer springs.  Start with the standard and adjust if needed.  If you are using a 16 inch barrel, you won't need the HBs and XP springs.  With mine I have only needed that heavier than standard buffers with the short barrels in the 6.750 to 10 inch range.  You also will not need an adjustable gas block.  If you are wanting to shoot sub-sonic ammo with a suppressor then an adjustable gas block MIGHT come in handy, but most do not use them even when shooting suppressed and sub-sonic ammo.  I do not shoot suppressed but I do shoto subsonic ammo as well as super sonic ammo in barrel lengths from 16 inches down to 6.750 inches and super sonic ammo in barrel lengths up to 28 inches and I have not needed an adjustable gas block yet.

I have one; great cartridge.

You don't need to buy Tomix brand; you'll spend a few hundred more than you need to if you do.

This is a cartridge that you really need to handload to enjoy fully. So many good projectiles to choose from, and you can select bullets that will penetrate an
engine block or that are frangible on soft tissue. One of our colleagues here just pointed out the performance of CFEBLK powder (Hodgdon) with the 458.
I'm a convert to it.

There are a bunch of YouTube videos about fine tuning the 458 SOCOM, and they have a wealth of info.

I recommend metal GI mags with removing a small concave shape at the front to prevent the case mouth from hanging up (it's not as much a problem with polymer mags).

The reason for metal mags is that the 458 SOCOM will make the polymer material bow outwards if you leave your magazines loaded for long time (like 6 months to a year). It can make it impossible to insert them into the mag well if that happens.
I load 10 cartridges in a 30 round mag, and have 2 mags mated together with a (metal) coupler.

If you 3D print, make yourself a few followers for the magazines. You'll be amazed how much easier it is to load them with the new follower. Thingverse has a dozen of these, and some are specific to magazine brand.
(If you 3D print, look for an interesting video on YouTube about hardening/smoothing prints by baking them encased in salt; not a bad technique for firearm related stuff, including frames)

Not a great cartridge if you don't handload, but a fantastic one if you do (BTW, it's a good cartridge to start your handloading hobby with too; pretty forgiving).
It's a 45-70 in a can.

Definitely buy just an URG, probably a Tromix. I’ve got their 8in barrel, receiver, and bolt that I assembled, and I honestly don’t see any need for a brake/comp for this round. With my Aimpoint and loaded mag, it sits right around 7.5lbs. Recoil is mild standing, and tolerable from the bench. It’s a great close in round, and it blows apart black bears with ease. People successfully use them up here on brownies over bait and in a stand.

Which you don't have to do if you buy quality (TROMIX) from the get go.  It will come fine tuned and will run like a sewing machine right out of the box.  This has to be some of the worst advice I have seen given in a very long time.  JMO

Look, I get it: Tromix is your girlfriend. Fine.

My budget 458 runs perfectly with off the shelf parts. Probably $400 total for the entire upper including a carbon fiber handguard from eBay.

After painting it probably took an hour to build (I milled open the ejection port before painting).

The 458 SOCOM isn’t some great enigma. It operates just like every other AR15 clone. Something doesn’t work, ordinary trouble shooting fixes it.

A lot of great info here, thanks to everyone.

That sounds like a rocket! Got a pic?

I agree, recoil is much exaggerated by some guys who like to enhance their chest hair with extravagant claims of recoil.  Brakes work and I use a lot of them, but they come at a price in concussion which on short barrels can be annoying, especially if shooting under a roof.  On my barrels less than 12 inches I use a MI flash can and a Limbsaver on a CTR stock.  Linear comps like the TROMIX and Noveske Flaming Pigs (modified) work too but add a lot of unwanted weight.  Even with 500s at 1050 fps out of a 6.75 inch barrel recoil is more than manageable from a bench and anyone without a shoulder injury can handle the recoil standing.

I hear the 405s are the worst in terms of recoil.

Naturally, that's what I loaded. I get 1,600 FPS with mine. It's stout, but nothing crazy.

It does start to hurt after a few dozen rounds.

How a classic .45-70 type loads (say 405's at 1300fps or so)?

I grabbed a used upper from the EE.
https://www.ar15.com/forums/equipment-exchange/458-SOCOM-TROMIX-upper/159-2144790/

For the near future I'll be buying ammo not loading. I know of Black Butterfly and Underwood ammo. Are there any other sites to check?

Pricey but SBR makes good ammo also.  Black Butterfly and SBR I know took great care to produce top notch ammunition.  I have not heard much about Underwood one way or the other.  I think if I were buying factory loads I’d go with Black Butterfly because I know they consulted a great deal with TROMIX to assure they were bringing out the absolute highest quality of .458 SOCOM ammunition.

Buy Tromix or RRA or don't even bother getting into the caliber.

I bought my barrel, upper receiver, and BCG from Tromix.  Runs like a champ (12" SBR.)

I never bought factory ammo - just reload.  Relatively easy.

Amazing power - I shot cinder blocks with 458, 5.56, and 9mm ARs.  No comparison.

Buy a complete upper from Tromix.

458S is not like all the others.  The guy who said that, above, is wrong.

I had the chance to run the BIG GRAINS 458 from Freedom Munitions recently. Ran great and at the time, it was the cheapest priced ammo I found. Unfortunately, it's out-of-stock right now, but try to keep an eye out for it.

Thanks for the link.

Looking for a cheap scope. How well with this hold up?
Primary Arms SLx 1-6x24mm SFP Rifle Scope Gen III - Illuminated ACSS-5.56/5.45/.308

I have quite a few PA scopes.  Mine are the ‘Classic’ 1-4 and 4-16.  The scope you mention is a cut above my scopes by a good amount and mine have not given me a lick of trouble.  I have 5 or 6 Weaver Kaspa 2-7 scopes on .458s and .375s and have had no trouble at all, and they are way cheaper in quality the the PA scopes.  IMO you would be well set with that particular scope.

Thanks for all of your help and info Big-Bore!

I have a Leupold VX II 1-4x20 on a .375 H&H. Recoil is a lot harder than a .458 SOCOM. It's got probably 750 rounds through it. I almost never take the scope off of 4X. 10/10. Would buy again (or it's current equivalent).

I agree, any Leupold is good to go.  I have several 1-4 and 2-7 VX1 and 2 on various rifles including .458S and there is no way you can go wrong with a Leupold.  One scope that has had my attention for along time but yet I never seem to get it, is the 1.5-4x Leupold VX Pig-Plex, for $300 from Midway, up $100 from this time last year.

 Honestly though, I think the most versatile power range is 1-2x on the low range and 6-8x on the high end, so that PA 1-6x is going to get the job done nicely.  I have a Leupold VX3 2.5-7x on a Ruger No.1 .416 Rigby and with 325 gr. bullets (man I wish Barnes would bring back the 325 gr. .416 TSX bullet) at almost 3000 fps, even at the relatively low power of 7x I have no trouble at all keeping 5 shots in a 8 inch circle at 500 yards (from bench of course).  High magnification is nice at the range, but it certainly is not the end all many act like it is.  And I bet more than a few critters owe their lives to hunters who have these high X scopes on their rigs when the shot presents itself at spitting range and the scope was left on high X.

I would love to be able to afford a Tromix but my budget wouldn't allow it...  I used a KAK lightweight 16in 458 barrel and A2 style flash hider, Brownells 458 BCG, a 458 cut stripped upper (don't recall which manufacturer) and a 15in Samson Evo rail.  Topped with a Trijicon RX01NSN...

I had to blend/polish the feed ramp to the barrel extension once completed it feeds flawlessly.  

I reload my own ammo and check each round with a case gauge.  

20180101_154300 by sky_soldier_82nd, on Flickr

Does a great job on NC Black Bear....

20191216_133010 by sky_soldier_82nd, on Flickr

20191216_132959 by sky_soldier_82nd, on Flickr

Sorry, your just wrong.  I've had a .458 with an upper not from Tromix and never again.  Not saying that people might not get lucky and end up with one that works but why take the risk?  I was having nonstop feeding and jamming issues from the chamber being out of spec due to being reverse engineered.

I would at least buy a complete upper. Your call to build or buy a lower. You will need to reload for this caliber, very little factory ammo available and is expensive.