Smith & Wesson has just expanded its M&P15 line with the .300 BLK AXE pistol and SBR.Suppressed, short-barrel AR-15s and .300 Blackout are a match made in heaven, and that’s exactly what Smith & Wesson is offering with its new M&P15 AXE models. They’re available as either a factory SBR or as a braced pistol, but both versions feature 8-inch barrels and are optimized for use with suppressors.That suppressor optimization is thanks to the built-in Gemtech GVAC technology. It adds an expansion chamber that allows excess gas to vent forward out of the muzzle, significantly reducing the gas blowback your…