Ever since I read TonyRumore's post about the .375 SOCOM, I have been fascinated with the thought of necking this monster of a casing down.  I have been wondering what kind of performance could be achieved if one were to neck the case down to a .308 caliber.  

I realize that the case may have to be shortened, and the shoulder pushed farther down in order for the cartridge to fit into the AR-15 magazines, due to the possibility of the bullet being longer that the .458 caliber bullets.

I had looked at the capacity of the .458 SOCOM, and found that, according to Quickload data, it has a capacity of 61.6gr of water, while the .308 Winchester has a case capacity of 56.0gr of water.  Even taking into account a slight shortening of the casing, and it being necked down to .308, I would think that a modified .458 SOCOM casing would still have a capacity that is close to 56gr of water.

What concerns me is the pressure that this round could generate.  While the 5.56mm chamber is proof tested at  a bit over 62,000 psi, I am worried that the metal that is removed from the barrel in order to accommodate the .458 SOCOM casing would require a much lower chamber pressure.

Does anyone know what the proof test chamber pressure of a .458 SOCOM is, and can an AR-15 barrel that has a chamber for a SOCOM casing take the pressures that would be required to launch a .308 caliber round from this size casing at speeds similar to the .308 Win?  

the issue is not the barrel it is the bolt face . alot of material is removed to accomodate the 458 rim so max pressure is around 35,000 psi.

If you really want to launch a .30cal bullet out of a big case your best bet would be a shortened version of a case in which the parent case was 30-06. The 458 case just isn't designed to be a high pressure case. Like mentioned above the bolt has been modified from the original version to fit the .473" rim weakening it. You are also correct about the chamber being able to hold the pressure. I ran the numbers quite a while ago and have since forgot the conclusion. But the gist of it is that there is quite a bit more surface area on the socom case that there is on the 223 case, which means that if you were to run a socom sizes case at the same pressure (psi) as a 223 case the force exerted on the walls of the chamber would be like 4x the amount (exact number have escaped my memory) and would likely be catostrophic. That's why the huge case of the socom has had its max psi set so low.

So, if you were to use a case based on the 30-06 the diameter is much smaller (.470" @ the base) compared to the socom (.540" at the base) would be able to jangle more pressure and put less stress on the chamber and bolt. Also, another thing to think about is the taper of the cartridge. The more taper the more bolt thrust there is. Less case taper means less bolt thrust which is good for our relatively weak bolts.

And the third thing to think about is that the socom case is close to $.80 a piece when mill and range pickups of a 30-06 based case can be had for under $.10 each.

Food for thought

What's wrong with the current off the shelf offerings? Ie. 30rar or 300 ossm (mimic's 30-06 ballistics with 150gr bullet)

That was something else I was going to suggest. Plus you have to factor in the cost of custom everything. Chamber reamer, dies, bolt (kinda). Off the shelf offering would likely offer more reliable everything. I think there is a 30 PPC and I know 30BR and their variants....

The bolt lug strength is the actual reason, the chamber will handle the hoop stress just fine - we have had adventurous folks load to pressures in excess of 70,000 psi and the chambers/barrels have held up without issue.  The bolt lugs, however, not so much.  The problem is a simple matter of capacity - shortened '06 cases (which the '08 is nothing more than a shortened 06) need to be driven at higher pressure to get the same results as the bigger case.  Shorten it even more and you need to get that pressure up even more.  The bolt will not take that abuse.  The SOCOM case, by the way, will take far more pressure than what the max is set at, bolt gun shooters will load to much higher pressures to really wring some fps out of the stubby 458 bullets.

Necking the SOCOM case down, due to the pressure limitation, results in performance that is not worth the effort - the closest to your desired performance is the 308 OSSM, which requires custom bolts, carriers and extensions.  Note that the WSSM and OSSM cartridges operate at the higher pressure and the larger case diameter without causing any issues to the barrel - again, the hoop stress is higher but not above the yield point of the steel....

I thought I was reading a post by Big-Bore for a mi ute until I looked at the avatar. Lol.

Well there you go. If you didn't believe anyone else, take it from Marty, his word is as close to gospel as your gunna get.

Who do think was my mentor?  See, I'm not making this crap up when I pen stuff; I'm just regergitating what Marry, Tony, and others have told me over the last 5 plus decades!

Besides the 300 ossm

you can look into the new 270 AR round (head of to where 6.8'ers hang out)

or 30 remington AR, and there is another called something that is similar to the 30 herritt (sp?) except it is a 6.8 opened up to 30 caliber.

Don't forget the 30 Grendel whatever they're calling that.

30 HRT. That's another Marty round.

The 6.8 development team actually tried any and everything between 22 and 30 cal, so they really did the original work.  Plus if you look up the 30 American, you find that was a factory offering long long time ago that was meant as a rimless 30-30.... never gained popularity though.  I just wanted to find a way to make the 300 Fireball barrels produced by a certain company usable, considering they were building them with carbine gas systems - not the way to go for the 240 SMK subsonic loads.  The 30 HRT is a cheap and easy conversion, dies are easy to find, brass is easy to find (nowadays, not back then, I bought the last 100 pieces of the 30 American brass that could be found, before the rest was used in the SPC development....).  The 30 HRT reamer cleans up the 300 Fireball chamber without issue.