Can anyone tell me why the 458 SOCOM round, even though there is no SAAMI spec, is generally considered limited to 35K PSI loads, while the 5.56 which it is patterned on goes all the way to 65K PSI? Is it the brass or some other aspect of the round? If it were piston operated could it go higher? If someone devised a magazine and barrel for an AR-10 platform could it go higher pressure for better ballistics? I have seen the "mutant" halfway AR-10 and it seems to limit the pressure to 35K PSI also. I guess I am asking what can be done to provide a platform which would provide better range for the 458 socom round. I have not looked at bolt rifles because I prefer a semi automatic. The 375 SOCOM goes part of the way there but still is AR-15 based.  Anyway, I want a 458 caliber gun.

35K use to be considered the upper limit but that has been revised to be more like 36-36.5 K psi.  I has to do with back thrust on the bolt.  A very simplistic explanation is that the back thrust on the bolt the area of the case head multiplied by the maximum chamber pressure.  With the larger area of the .458S case head, .308 sized, at 35K psi, the same amount of back thrust is exerted on the bolt lugs as it is with the smaller 5.56 operating at 55-60K psi.  It is a little more involved than that but this gives you an idea of why it was limited to 35K psi.  Tony and I have been pushing the .375 SOCOM, same .458 case necked down to .375, to 36.5 K psi and with over a thousand rounds through my rifle alone, I have not seen any ill affects so the 5.56 bolt lugs are stronger than they calculate out to be.  Also, it turns out that most of the loads we have been using as max over the last decade and a half turn out to be in the 36-37K psi range, not the 35K range we estimated.  And the bolts do not give up the ghost and the brass is extremely long lived.  Another limiting factor is the use of pistol primers.  They have thinner cups than do rifle primers and they generally operate at pressures below 44-45K psi and start to flatten out at those levels.  Rifle primers, as you know, easily take 60-65K psi before they start to flatten out.

On another note, the velocity gain going from a 300 gr. bullet at 35-36K psi to 40K psi is minimal.  Once you get into that range, a very small amount of powder really ups the pressures but you do not get a corresponding upping in velocity.  If you look at the pressures they run in that hybrid rifle they are much higher than in the AR-15 but if you then look at the pressures they are actually getting, they are not that much higher.  Higher yes, but if you calculate out their down range performance, the gain is not really worth talking about.  

Also, none of the SOCOMs use any kind of special steel or special heat treatment of the bolts.  They are just opened up, the ejector and extractor are modified slightly, and they are good to go.  They do not even need to be re-heat treated after opening up.  Although that company has implied that .458 SOCOM bolts are failing left and right, that is an outright falsehood.  I think if they were breaking all over the place, Rock River, TROMIX, SBR, and Teppo Jutsu would have heard about it.  Another case of a company making false accusations to justify their proprietary product at twice to three times the price of the standard product.

I know you said you do not want a bolt gun, but in bolt guns the .458 SOCOM is easily taken up to 50K psi.  What we do is ream the primer pockets for the thicker LR primers and load away.  My own bolt pistol, a Striker, still is using LPM primers but I only have pushed the pressures to where the primers are flattening out quite nicely, likely in the 44-46K psi range.  If I were staying with this load I really should ream and load with LR primers but I have not done so, and likely will not because I am backing off on this load due to the excessive recoil.  Arthritis in my hands is making it too unpleasant to shoot anymore so I need to back it down and switch to a lighter bullet.  Bummer.  The brass is lasting just as long as my AR .458s though but the recoil is such that I really do not care to take it to the next level, or even stay at this level anymore.

Thank you. Very helpful info. Recoil is and issue for me too. For that reason I am seriously considering converting my SOCOM pistol to SBR.  One more question. In my 300 blk guns (I have several) I use #41 primers because I am concerned about slamfires. In fact I use them in my .223 reloads also. If I were to ream out some brass, could you recommend a nice hard LRP?  Would a mil primer for .308 ignite the powder well enough? Thank you very much and have a great weekend.

I see no reason to use a super hard military primer.  They would work just fine, but nobody I have ever heard about has had any trouble with CCI, Win, Rem, or any other LP primer I can recall.  The only primer I would not personally use, whether for factual reasons or not, are the Federals.  I know from decades of using them that they use softer primer cups than all the others and honestly, they scare the hell out of me in a semi with a floating firing pin.  I only use Federals in bolt, single shots, and revolvers and there they have no peer IMO.  I love them.  However, they show signs of flattening at start loads that the unfamiliar could interpret as being high pressure signs, so you cannot read them like you do all the others.  That's why I know they are soft so they will not be used in any semi with a floating firing pin, not by me anyway.

Since I have always be under the impression that in terms of 'hotness' regarding primers, magnum primers are hotter than standard and rifle primers are hotter than pistol.  If you reamed out the primer pockets to take LR primers, what do you gain?  Nothing.  You still cannot exceed 36.5-37K psi or you could suffer bolt lug failure from excessive back thrust.  And by using pistol primers, if you see them flattening you know you are already well over 37K psi, likely around 44K psi, but with a rifle primer you could be pushing 65K or more and your rifle could turn loose on you without the primers ever indicating that things were getting a little hot.  Little hot hell, a whole lot hot!

Another thing I have always read about is that the use of magnum primers is like adding another grain of powder.  I'm not sure I'm buying this because I have substituted standard primers in place of magnum primers and saw no change in velocity (going from magnum to standard, I have never tested going from standard to magnum).  Maybe if it were really cold I would have but still, "they" say reduce the maximum load by 1 grain if it was worked up with standard primers and you are loading magnum.  Better safe than sorry so I do it.
 I have also read that switching to rifle primers from pistol primers does the same thing.  I have never tested that but my own paranoia would say if the maximum load had been worked up with LPM primers and I was switching to LR or LRM primers, especially LRM primers (after reaming the primer pockets of course) I would either drop the maximum listed charge by a grain or two or use the chronograph and stop when the velocities were the same between the load using LPM and LR/LRM primers.  I would not try to exceed the velocity you were getting with the LPM primers.

I have always heard that CCI primers use the hardest cups with Winchester being a close second.  In ARs and LRs, I always use the standard CCI primers, by standard I mean non military.  In my 6.8s and their wildcats I use SRM, the 5.56 uses SRM, SOCOM based rounds all get the LPM, .338 Spectre (10mm magnum based) gets LPM just like the SOCOM rounds, and my LR-308 sized wildcats and .338 Federal all use LRM primers.  Sure, I get primer pecks when you drop the bolt and chamber a round, but I see that even on military milsurp ammo and the peck does not look any more shallow to me with the military primer than with the CCI primer.  It certainly is not enough of a peck to give me any concern and I am a pretty paranoid person.
 
In a bolt action where you can safely push pressures to 50K psi or more, sure, ream away and use LR or LRM primers, but in an AR I personally would not do it.  It may lead you into a false since of security with your loads and that could get you into big trouble very quickly.  Using the LR primer is not going to allow you to shoot hotter loads and IMO they really do not reduce the risk of slam fires.  I've dropped the bolts on thousands and thousands of rounds of SOCOM ammo and never had I had a problem with a slam fire or anything close to it using pistol primers.  And if you practice safe gun handling, having a slam fire is not going to do anything other than scare you shitless.  Always assume when you chamber a round in anything that it is going to go off.  Do that and then if and when it ever did happen, no big deal.  Under ware can be washed.  

The only thing I have found in regards to the use of pistol primers, I think every one of my ARs in a SOCOM based caliber has had an issue with intermittent primer piercing, sometimes even on mild loads.  In every case the firing pin was within mil-spec, .028-.036" protrusion, but IMO a firing pin over .030 is too long for the softer and thinner pistol primers, and that causes the piercing.  I use the standard firing pin until it pierces a primer, then I buff it out to remove the erosion and set the protrusion to no more than .030 inch.  After that, no more pierced primers, ever.  I have never had what I would consider a safety issue with pierced primers other than the pierce, no blow back, no bolt face erosion, nothing, just after shooting I'll notice a burn through in the primer dimple and of course, the firing pin tip will be eroded if you look at it under magnification, even with just one piercing.  Rifle primers might prevent that with .032-.036 protrusion firing pins but for the reason I mentioned above I would never use them in the AR.

You have been more fortunate than I in respect to slam fires. Out of approximately 50,000 rounds over the past 40 years I have had 3. Two on a .458 socom and 1 on a 300 blackout. No injuries or damage but very, very scary. All shooting in not on a range. In my work I have had to chamber a round in odd situations - in a hotel room, in a vehicle etc. not good practice but unlike deer, bad guys shoot back and a second can make a difference. I see the hard primers as just another layer of safety, not a performance enhancer. I will try some rifle primers and report back. Have a great afternoon.

In that case, ream for LR and I'd stick with standard LR primers.  You don't have the volume or difficulty of ignition with the powders suitable for the .458S.  I'd just work up the load to the same velocity that people get with LPM primers and call it good.  
Be sure to ream the pocket though because a high LR primer in an unreamed LP pocket would certainly invite a slam fire.