Those bastards at Primary Arms are having a sale on 458 SoCom components. Using the assumption of stripping out an fully operational 5.56 AR what parts would be needed to to build a 458?
I know barrel and bolt, anything else?
I have built all but one of my ARs so I have the tools and assembly knowledge down pretty well.

Muzzle device ,or thread protector and magazines.

Maybe a gas tube and block....

Magazines? Thought standard Stanag worked.
Of course brass. bullet molds and dies too. Looking forward to hunting Covid dinosaur zombies soon!

Bolt, barrel, appropriate muzzle device (or not) and ammo (duh) are the only items unique to a 458.  

They also recommend an upper with a larger ejection port, not required.

Got it. Everything is ordered. Looks like it is time to say goodbye to my old XM177 clone and hello to a monster hunter. I really don't need to spend this money but for less than 500$ I can't bitch too much.
I have a love/hate relationship with this place.

Enlarged ejection port, I think a standard upper would invite some FTE issues. The bolt is different, barrel, extractor is reprofiled. Buffer system is the same. ymmv with primary arms though. I’d stick to tromix for anything socom related.

DON'T DO IT!  Those Primary Arms .458 SOCOM barrels from BCA and Radical are for the most part garbage and so are their bolt carrier groups. They don't have a cluse on how to make a properly functioning BCG or barrel extension.  Don't be penny wise and pound foolish.  Buy QUALITY from the people who really know how to make .458 SOCOMs, not Johnny come Lately wanna be.  Go with Rock River, SBR, SSK, or if you want the best quality, best customer Service, and best price, made by the guy who started it all with Teppo Jutsu, TROMIX.  You cannot simply open up the bolt face and call it good on the bolt nor can you just grind away at the barrel extension a little and make it feed properly.  Do it right and you won't join the legions of those who have come onto the .458 SOCOM forums wanting to know why their $89 barrel won't feed worth a damn.

Do you homework.  And by the way, PA always has those .458 SOCOM Radical and BCA barrels on "sale" for $89 and the BCG for $100.  Their SALE price is BS.  They are always that price and use it to hook the unsuspecting.  

Yes, occasionally you get someone who claims their barrel feeds and shoots well, but most of those guys are extremely lucky or outright stretching the truth because they don't want to admit they got burned, or they shoot it one time or two times and put it in the closet to bring out and impress their friends with the old "Mines bigger than yours" BS.

YOU MUST have he ejection port enlarged or buy one already enlarged, such as from Aero or Anderson Arms.  They are both good to go.  You cannot put 10 pounds of crap in a 5 pound bag and that's exactly what you are trying to do when you try to eject a .458 through an ejection port sized for the 5.56.  You cannot even force it through the standard port.
Paladin Machine Shop Service, tp555(at)windstream.net, can enlarge your port for you if you have a specific upper you wan to use that does not offer an enlarged port version.  He will also lathe square the front of the receiver for you if you ask and do it all for very little money and a super fast turn around.

BULL CRAP!  Unless the port opening is already enlarged, as mentioned above, you cannot force the .458 case through a standard sized port opening.  Quite giving the guy bad advice.

BB knows what he speaks and it's the truth.

Well I did order an Anderson enlarged port upper somewhere. I think I bought parts from 3 different companies. I had no idea this stuff was junk but I did order it so I will go through with it and see what happens. All that is left is to get some 458 cases to reload. (figuring on Starline)

BCA 450 upper did this today at 100  About 5 seconds between shots.  Cannot claim 100pct reliability though but i bought it for $200 for a hunting trip. Attached File

Nothing wrong with the stripped .458 Anderson upper.  I have quite a few of them on my .458s and I have no complaints.
Starline is the only game in town for .458 Brass.  Yes, you can find it with other headstamps but it is all made by Starline.  Maker Bullets, when in stock, has it for 68 cents each.  That's about as cheap as I've seen it in 100 case lots.

About dies, stick with Hornady or Redding for the sizer die, CH-4D or Lyman for the expander die (I like to expand the case neck in a separate step because I firmly believe it is easier on the brass) and Hornady New Dimension seat die.  Redding or Hornady shellholder.  I use a universal decap die to deprime.  Also, save yourself a lot of headaches and use Imperial Sizing wax.  Other stuff kind of works, but nothing I have tried works better for sizing or forming than Imperial.  I think Redding now makes and markets it.
If you want to buy a die set, I recommend the Hornady and then buy the M-type die from CH-4D. I cannot stress enough how great the Hornady seat die is.  I have heard so many people complain about hard bullet seating and almost always it's because the bullet is not starting straight.  The Hornady die with the alignment sleeve starts bullets straight and makes seating so much easier and you would be hard pressed to crush a case with that seating die.  However, if you crimp with the Lee Factory Crimp die.  A roll crimp, if not adjusted exactly and must have the mouth and crimp groove aligned, will crush a case in a heartbeat.  The Lee FCD won't.   Although, I have never had a .458 load that actually needed crimping to keep the bullet from moving in or out under recoil.  There is not that much recoil so don't worry about it.  However, the LFCD will almost always lower ES and SD and that theoretically should shrink groups, but you can't prove it by me.  Still, I like small ES and SD on my loads so I add 1/4 turn of crimp with the LFCD on almost all loads unless group size starts to open, which has happened to me but it is rare.

This is the only thing he is fibbing about, OP.  Sent FW_wife to Montana with my brother's Rock River .458 upper when she went to scatter Fatalwishes' ashes.  Threw it on the lower he had built for her and sighted it in.

This is not the caliber to go lightweight on.  As big a buttpad as you can manage, perhaps lead weights, and consider the Israeli buttstock that has a spring.

I will be casting my own bullets. Such a low velocity round should be easy to work with. Not even going to bother with gas checks.
Is recoil bad enough to require anything special in optics?

Not fibbing at all.  If you use an A2 butt stock it will eat you alive.  Use a LimbSaver on a Magpul CTR type stock or a Kick-EEZ on an A2 type stock along with a 4 port TROMIX muzzle brake and you won't need extra weight to carry around to make it a very low recoiling rifle, even with 500 gr. stout loads.  Yes, it kicks more than a 5.56 or 6.8, but it is nowhere near as bad as a lot of people try to make you think it is.  The only people I have heard that have actually shot one and found the recoil painful are those using an A2 type stock with standard meat tenderizer butt plate.  Those are brutal.

As to optics, you would not believe some of the cheap Chinese optics I have on some of my .458s and they have given me no trouble at all.  I have several Primary Arms red dots and they have not hiccuped at all.  The PA 1-4, while bulky as the dickens, has held up just fine also.   The cheapest scopes I have tried are the $79 and $89 Weaver Kaspa 2-7x and 3-9s optics.   Now those are undoubtedly about the cheapest "name brand" scopes you can find and with many hundreds of rounds through them they have never balked.
As a note, my 6.750" .458 pistol is very light.  I cannot remember exactly what it is but its well under 6 pounds, probably under 5 pounds, and it has a PA AMD on it.  I shoot nothing but 400 and 500 gr. bullets through it, both at 1050 fps, no brake, no stock (SBT A3 brace), only a MI flash can so its recoil is not softened at all.  I likely have 800 or more rounds through it and that little Chinese made AMD has taken it all in stride.  If that won't shake up an optic, I don't know what will.
You didn't ask, buy my favorite power on the .458 S is 2x-7x and favorite scope is the Leupold VXI or VXII, which ever you can get on sale and cheapest.  I have both and while they are not going to impress anyone, they have great glass and work very well.  If you cannot see an animal well enough at 7x  and within its range, 200 yards or less, then IMO you should not be shooting, and at 2x you have plenty of field of view for that shot that comes measured in feet instead of yards.

Oddly, there have been a few to say their Nikons have failed but I have a buddy who swears by them, and yes, he has one on his .458.  You don't need to spend big bucks but good glass is always money in the bank IMO.

One more comment about cast bullets.  I have shot cast bullets for over 50 years through every type of action you can imagine.  I have always been able to get at least 1.5 MOA for 5 shots at 100 yards with any rifle and at least 4 MOA  (2 inches at 50 yards) with my pistols (.45-70. .50 AK., .454 C, .309 JDJ, .44 Magnum, .45 Colt...), but I cannot get any cast bullet to shoot worth a tinker's damn in any of my ARs or LRs.  I want them to be at least 2 MOA for 5 shots at 100 yards and I just cannot get any of them to do it CONSISTENTLY.  Sure, I get frequent good groups that would make anyone happy but just when I think I have found the secret, things go to hell in a hand basket and open up to 3 or 4 MOA.
Gas checks, different lubes, over sized .001 and .002, seated to the lands, seated off the lands in many increments, you name the trick and I have tried it.  Bullets that shoot incredibly well in my .45-70s, both pistols and rifles, shoot all over the place in the ARs.  Now, when I single load them, accuracy is much improved, but if I want a single shot I'd take the Ruger #1 .45-70 or at least the LA .45-70s.   If it won't shoot accurately in semi-auto, screw it.  Maybe you will have better luck than I, but don't be surprised if you don't.  I have tried for over 15 years to get the .458 S to shoot cast to my liking and it has not happened.  I gave up a couple of years ago and won't try anymore.  
If looking for a blasting round and 3-5 MOA is not an issue, that's easy to achieve.  But less than 2 MOA consistently has proven impossible for me.

Also, I have never shot a plated or powder coated bullet so look to others for information on shooting them.

Big Bore, Thanks for the input. I gave up chasing accuracy, My eyes won't let me anymore. I did get an AR in .300 whisper (yes, whisper, long before this blackout silliness came about) to shoot well. Gas checked "castboolits" 247gr bullet. Loads of fun. The biggest reason the 458 got my attention was years ago I met a guy with one, new fangled and all fancy back then. Now it is my turn to give it a try. I have also been toying with the idea of suppressing it. Gotta admit, a 405gr suppressed subsonic sounds like fun. I can handle recoil fairly well but I did sell off my John Ross .500 S&W because my wrists could no longer handle it. If necessary I can dig out an old recoil pad. A 458 rifle can't be worse than a sporterized M39 Nagant or H&R single shot in 3 1/2 magnum (double and triple ouchies!)
I will definitely keep all you have said in mind as I go through this.
Do you think a muzzle break is worth trying? I know it is a low pressure round but would it make a worthwhile difference?

My brother sample chased suppression in .458.  Got a Titanium Lane(?) suppressor for it.  I gave him a couple of boxes of subsonic 500 grn .458 for his groomsman gift.

He reported that .458 SOCOM suppressed was much louder than his suppressed 7.62x39mm.  Big bore, big hole through the suppressor, a lot of gas.

Some brakes work well, others don't.  I am a big fan of Shrewd brakes on high pressure/high velocity rounds, sort of a radial compensator, but they don't work worth a tinkers damn on the .458 SOCOM.  The TROMIX 4 port fish gill works the best of all I've tried.  The Midwest Industries 3 port work great but it must be opened up to clear the .458 round.  The Noveske KX3 and 5 are nothing but blast diverters as is the MI flash can, but they are not designed to be brakes in the first place.  I've tried a couple of linear comps and they do little to reduce recoil, acting more like a blast diverter.  The TROMIX TANK brake works very well but IMO not as well as their 4 port fish gill brake.  The 4 port TROMIX comes in two sizes, standard and streamline.  Personally, I like the looks of the streamline better.  I have both and they both work the same, so it's a personal preference on which to get.
 
In short, from what I have personally used, if a brake is designed for high pressure high velocity rounds, like the Shrewd, then it might be the best choice for reducing recoil with the .458 S.  If its designed for big bore low pressure rounds, like the 4 port TROMIX and TANK, then it should do much better.  If you like the corn cob look of the MI 3 port, it is a great brake also but you will have added expense having it opened up to clear (Paladin Machine Shop Service can do this quickly and very little cost-tp555(at)windstream.net).  While it is designed for .308 rounds, once opened up it works very well on the .458 also.

If not looking for a brake and you're not going to need a suppressor mount, one of my favorite flash suppressors is the YHM Phantom, .30 cal, but it too will need to be opened up for the .458.  However, it is an easy job to do yourself.  The metal is not especially hard and the front half is bored 1/2 inch so will act very much like a guide to a half-inch drill bit for you to drill through the base and enlarge it to clear the bullet.  I've done quite a few of these myself with the FS held in a vice and using a hand drill and lots of cutting oil to drill them out to clear the .458 bullet and have excellent results with no loss of accuracy.  Needless to say, but I'm going to say it anyway, do NOT drill the FS out with it on the rifle.  You'd be sure to go through and hit your muzzle crown if you did.

What BackBencher said is true.  The larger the bore, the less the suppression.  That is why I have never gotten a suppressor for mine.  I have almost pulled the trigger on a VERS 50 for use on my .500 Phantom and .458 SOCOMs but after listening to a friends .458 suppressor, I was not impressed enough to drop over a grand and still have to wear hearing protection while hunting.  Besides, electronic ear muffs are legal in my state for hunting deer and they really help me hear them before they hear or see me while letting me keep my hearing when I get to shoot.

How does that stack up to a .45-70?

Linear compensator will never reduce recoil, only sound to the rear of the compensator.  Same as a flash can, but made of steel, smaller and heavier.

IIRC, Trapdoor .45-70 ballistics are similar to .458 SOCOM, albeit at lower pressures.

The .45-70 and .458 SOCOM are very close to equal in bullet weights up to 300, maybe 350 gr.  My .458 loads are within 100 fps of my Contender and Marlin lever action loads.  But heavier than that and the .45-70, AKA God’s chosen sledgehammer, leaves the SOCOM in the dust.

I am a huge .45-70 fanboy and the SOCOM’s performance being so close to the .45-70 with the bullets I hunt with is what attracted me to the .458 S to begin with.  It’s been a love affair going on almost 20 years now.  My how time flies!

Listen one of the only guys to listen to on here about 458 SOCOM is Big Bore.  Make sure your ejection port is enlarged and buy yourself a bolt and barrel from Tromix.  I called and got ahold of Tony and he answered all my questions.  I hung up and ordered off of his website.  Don't make a mistake and cheap out on the barrel and bolt.

@Big-Bore I've been having a bit of trouble with .458 socom since I got into it about a month ago. I have a 12" Tromix upper, so that's good, but for the life of me I cannot get any sort of groups under 3 MOA with any load. I've seen data where the 325 FTX gets 1 MOA groups at 100yd but I've gotten as high as 6 MOA using the exact same loads as what's in the data. any idea what could be the problem? I don't even care about velocity at this point, I'm just trying to get something that hits where I point the gun.

The only things left worth noting are
1. I'm using a TA01 ACOG. Maybe it's getting thrown off from the recoil? Doubtful considering others are using $150 scopes and doing just fine, but I figured it was worth noting.
2. For some reason all of the bullets have this ring around them about halfway between the tip and the cannelure once they're seated. I thought it was because of the crimp part of the seating die was way too dialed in, but it's backed out as far as it can go, and I use a completely separate die to crimp, if I choose to do so. I still don't know what's causing it.

Number two is easy.  That ring around the bullet is caused by the seating stem not fitting the bullet properly and is causing the ring when seating.  If that ring is concentric all the way around and even, it is of little consequence.  It may look like crap but it won't affect accuracy, not within the ranges we use the .458 SOCOM.   If however, that ring in more prominent on one side than the other, that is caused by the bullet seating crooked at the start.  The seating die I recommend is the Hornady New Dimension seat die, .45 caliber of course, along with the FTX seat stem which you need to buy separately.  That should eliminate the ring around the bullet and also assures the bullet is seated straight with no cant.
 
Number one is a bit harder.  First of all I doubt it is the optic but to rule that out, can you switch to another optic just to make sure it's not the optic or mount?  I had a thousand dollar Leupold on my .50 BMG for years and nary a problem, then on my .30 HRT the scope went tits up during load work up on a new bullet.  Also, make sure the optic's mount is not touching the handguard and is fully mounted on the receiver. With the ACOG I doubt that is your problem either though.

 Second, what kind of handguard are you using?  If it is one of the skinny handguards then you really have to make sure the gas block is not hitting the handguard and that there is at least 1/16 inch of clearance all the way around it, especially at the bottom.  IIRC if that is the standard barre then that barrel may have a .875 gas block and skinny handguards often contact the gas block on the bottom.  The gas blocks I use are TROY because they have the thinnest bottom of any I have used.  Others, even though listed as low profile, I have had to thin the bottom down on a belt sander to give me enough clearance.
 
Next suspect is the muzzle device.  What kind are you using?  If over tightened or too loose it can cause problems or if the bullet is making contact with the muzzle device then that would certainly give you patterns instead of groups.  Generally the 325 gives very good accuracy although I have never used them.  
If none of these apply and you are dead certain, try shooting it without a muzzle device and see if that tightens groups.  
IF the barrel is tight with at least 30 ft-lbs of torque, the gas block is not even close to hitting the forend, and the muzzle device is not too tight or contacting the bullet, and another scope does not tighten groups up, a call to Tony might be in order.  My 12 inch barrel is the lightweight with a .750 gas block and is my favorite length and you should be getting a lot better accuracy than that.

I'm using the midwest 11" handguard offered by Tromix. The upper was shipped to me assembled. I took the flash hider off in favor of a SiCo .46 caliber muzzle brake to fit my Hybrid. I've fired it with and without the hybrid, and haven't seen any difference. Bad groups all around. I'll see if taking the brake off will do anything. The best groups I've shot were 2-2.5" with hornady 300gr JHP/Accurate 1680/WLP.  I think my last ditch effort is going to be buying some factory SBR short barrel optimized ammunition and see if that works. I haven't fired any factory ammo through the gun since I got it. Thanks for your help!

^^^^^^^^^THIS!!!!

This man knows what he is talking about!!

Love my RRA 458 Socom loaded with extreme penetrator rounds,shatters AR 500 plate like fine china .

Did you work up your loads or just go off max charge weight? More often than not your barrel will like a specific charge weight that is nowhere near max. I think my 405gr load I settled on was 34gr Re-7, which if I recall correctly was closer to minimum charge weight, but accuracy was great and my SD was in the single digits.

You say it if 46 caliber.  I think it is safe to assume that the maker says it is good for up to 46 caliber and the opening is around a half-inch.   Since it gives the same poor grouping with and without the suppressor, I suspect the brake is causing the issue, if it is the brake, and not the suppressor per se.  Try removing the brake before buying that gawd awfully expensive SBR ammo.  They make great ammo but you should be getting better accuracy than you are with your home brews.  Let's try all the cheap solutions before spending more money. And make sure the brake is not over tightened because IIRC suppressor mounts do not use a crush washer and butt up against the shelf behind the threads.  Often people will want to index the suppressor and get a little ham-fisted on the mount trying to get suppressor's writing in a certain position.

 And just for kicks and giggles, double check that the gas block is not coming near the handguard.  I love MI handguards and that's all I use but if your gas block has a fat bottom it can hit the handguard, especially if it's a .875 or .936 gas block.  I had to thin more than a few fat bottomed .875 and .936 gas blocks or file down the set screws to keep them from hitting the handguard.

And don't forget swapping out the optic to rule the ACOG out.  They can fail just the same as any other optic and I've heard of more than a few failing over the many years they have been out.  They are tank tough but they can fail so rule that out before spending money on other solutions.

Ok. I'll see if I can't get the brake off sometime in the next few days. That gas block is indeed pretty damn close on the bottom to the handguard, so maybe it's an "all of the above" issue

Well it is done and I took it to the range today. Recoil is a bit stout but not too bad. It functioned well. Fed and fired correctly. As for accuracy, no idea. It seemed to be doing ok but only for the first few shots. About round number 7 or so the TR25 red dot broke. Oh well, time to upgrade and try it again soon.
Load was 405gr hardcast (Lee mold) over 27.7grs of WC820.