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Changing Caliber In An AR-15


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Recently I had the 458 SOCOM brought to my attention.  I read up on it in Wikipedia.  It is interesting that the M-15 can be made to use this kind of ammunition.

Look up the write-up in Wikipedia.  It is lengthy and covers the subject better than I can in a post.  My take is that after obtaining a 458 SOCOM upper for your existing M-15 lower,  it would be best to buy brass and 'roll-your-own' ammo.

I personally,  if I did not already do my own loading,  would buy a Lee balance scale,  a set of dies,  a Lee "nut-cracker" hand press,  perhaps two hundred new casings and several boxes of bullets.  (Experimentation needs several bullet designs and weights and powders;  or take your best guess and use the powder of the Wikipedia article and see how a given 300 gr bullet does.  After you have one you & your rifle like,  buy 500 or 600 of that bullet and load them into the casings.)  There is a bunch of small items also needed that are quite cheap as individual items but collectively add up.  It still might be possible to be doing this loading for under a $hundred?  Exclusive of the price of the casings,  bullets,  powder?

The best caliber I know of for hunting feral pigs (called by many,  many names--  including the ones suitable for print) so far is the 44 Magnum.  A heat-treated hard-lead WFNGC ~300 gr bullet from a 44 Mag revolver (a big Ruger) or better from perhaps a 44 Mag Lever Rifle will do a fine job of stopping any pig.  The 300 grain bullets/loadings put too much stress on any but the big heavy-duty Rugers.  I fired 6 rounds of this stuff from my Smith and swore never again.  OTOH,  my Marlin 1894 Microgroove does its best with this 300 gr lead.

The M-15 platform in 458 SOCOM just might be the optimum 'pig gun.'  Instant availability of a lot of rounds if faced with a wounded pig in a 'situation,'  ability to shoot more than one pig with only the need to move the targeting,  a short quick-retargetable rifle.  The 300 gr flat-nose 458 lead slug in a hard-cast lead WFNGC heat-treated version, if available, would seem to be ideal.  Just need a report from someone who hunts pigs and who may have tried it.  The cartridge & rifle would seem to offer minimum recoil to the point of perhaps essentially no felt recoil,  ~2000 FPS,  a slug that would hardly notice the pig,  and real-world the pig would think it had been hit with an RPG?

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  • 3 weeks later...

To put more information in one place:

The 458 SOCOM is supposedly the result of a back-yard Bar-B-Que & beer party during year 2000.  Some attendees of this gathering were significant people in the military and in the firearms industry.  The objective was a minimal new-equipment firearm to 'stop' bad guys.  Like in Afghanistan.  The ideas worked out well.  It is supposedly said in Special Ops circles that "If you want to reach out and touch someone,  use a 5.56. Use a 458 SOCOM if you would rather reach out and tear them apart."  Or,  in our context,  stop that 250 pound charging boar in its tracks.

The details are dealt with very well elsewhere.  If you as the reader are interested,  some basic search will find a lot to read.

The result was to use a design for the brass that used the shell base common to the 308 Winchester and several other cartridges.  Casing length was kept where a common bullet would fit with COAL to match the capabilities/length of a standard 5.56x45 magazine;  casing max diameter to fit in that standard magazine for the M-4/M-15.  The 5.56x45 is essentially a double stack in the magazine.  The 458 SOCOM is a neat single stack.  Therefore,  the same lower receivers and magazines used for the M-15/M-16/M-4 can be and are used.  The same upper receiver is a start. There is a different barrel extension and some other machining. The same bolt carrier works but a special bolt is fitted.  All the parts in a standard M-15 lower are untouched and a 458 SOCOM upper will intentionally attach directly to an M-15/M-4 lower.  Note:  it is really best to add a recoil pad like the 'Limbsaver' to the stock.  This SOCOM rifle will leave bruises otherwise,  at least with some loadings.

I have noted in other posts that I added a Limbsaver pad to my Rem 700 in 7 MM Rem Mag and that recoil which had been severe went almost entirely away.  Limbsavers on my 308's,  bolt and AR,  make firing those about like shooting my Ruger 10-22.  If you shoot mostly in winter with a heavy coat it may not be a big thing either way.  I do much of my shooting on nice summer days when the local temp is above 100F and there is a worn T-shirt between me and the rifle.

Starline makes 458 SOCOM brass which is easily obtainable from most common vendors.  The bullets are standard for several nominal '45 caliber' cartridges using a .458 dia bullet.  Very specifically,  these .458 bullets are used in the 45-70 Government cartridges and the external ballistics of the 458 SOCOM are identical to the 45-70 Govt.  Bullet weights run from a high of ~500 grains with a very subsonic muzzle velocity,  well suited to Special Ops with suppressors and needing high stopping power,  to very respectable relatively flat-shooting rifle velocities using 225 to 300 and 325 grain bullets.

This cartridge uses large pistol primers and carries the warning that like the M-16,  the thin primers are susceptible to slam-fires.  Commonly available powders are used like Lil Gun,  RL7,  others.  Look up commonly used loads described in other forums.

Loaded ammo is available from all the usual vendors.  It does tend to be pricey.  If I started with loaded ammo I would want to have the capability and would reload my fired brass.  Some of that is reducing the cost of ammo but more is the fun of making my own and trying to beat the accuracy of factory offerings.  I have found a lot of copper-cased bullets and some hard-cast lead bullets but not any hard-cast gas-check bullets yet.  There was a listing for a ~400 gr gas-check bullet mold.  Not what I am looking for.  Hard-cast heat-treated bullets are more than I am willing to try to make for myself.  Want to buy a ~300 gr WFN gas-check hard-cast for feral pigs:  after and if I ever buy a 458 SOCOM.

Accuracy reported is excellent;  maybe spectacular;  out to at least two hundred yards.  More to read about in those other forums.

Uppers and complete rifles are made and sold by Rock River Arms and by Wilson Combat.  So far.  Components to build your own are also easily available.

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