SIG MPX is the official name of the new SIG Sauer submachine gun that has been rumored ahead of the 2013 SHOT Show. I have been in contact with sources at SIG Sauer, who have confirmed both the gun and the new name.
For those not familiar with the information on this new SMG, the SIG MPX will be a pistol-caliber sub-gun with controls very similar to those of an AR-15. The first run of guns will be chambered in 9mm, .40 S&W and .357 SIG.
A full auto version will be available, though restricted to government buyers in the United States. However, there will also be two civilian versions of the SIG MPX: a 16″ barrel carbine and a short barrel rifle (SBR) version with an unknown length barrel.
Pistol caliber SMGs have been popular with various military and law enforcement units, with the HK MP5 being the standard by which all other modern sub-guns are judged. However, I have seen a migration away from the MP5 to short barrel AR-15 carbines by many SWAT teams.
Only time will tell how popular the SIG MPX will be. However, I can see the .357 SIG version as fitting into a neat position between the MP5 and the AR-15. It would maintain many of the benefits of the MP5, such as size and less report than a SBR AR-15, yet provide more “stopping power” than a 9mm. Also, out of a longer (than a pistol) barrel, the .357 SIG cartridge is likely to defeat most soft body armor.