In 1957, Eugene Stoner was chief engineer of the ArmaLite Division of Fairchild Engine and Airplane Corp. and had been part of the team that fashioned the AR-10 around the 7.62x51mm cartridge. (It followed the AR-5, a .22 Hornet survival rifle for the Air Force.) Asked to trim the AR-10 for a 5.56mm round, Stoner came up with the AR-15. It became, in uniform, the selective-fire M16 and variants. 

The .223 Rem. — civilian rendition of the 5.56x45mm — remains the top-selling offering in AR-15s. You get more versatility from the 6.5 Grendel, developed on the 6.5 PPC Benchrest case by Bill Alexander and Arne Brennan in the 1990s. It sends 129-grain bullets at 2,500 fps. The 6.5 Grendel in a bolt rifle blessed Brennan with a 1.2-inch knot at 600 meters. While the Grendel’s 1.52-inch case is shorter than the .223’s, loaded length is the same. Les Baer’s .264 LBC AR is the Grendel’s twin. I’m also sweet on Remington’s 6.8 SPC (Special Purpose Cartridge), an under-sung .270-bore round that has impressed me as ideal for whitetails. Winchester’s .350 Legend, pitched to deer hunters in states that now permit straight-walled rifle cartridges in traditional shotgun-only areas, gets sunnier press. 

For deer and hogs in Southern thickets, there’s the .450 Bushmaster. It derives from the 6.5/284. Hornady loads 250-grain FTX bullets to 2,200 fps. Another big bore, the .458 SOCOM, owes its genesis to Marty Ter Weeme’s query to Tony Rumore re: AR uppers for the .440 Corbon and .50 Action Express. Marty rebated the rim of the .50 AE to .473. Manson Precision supplied a reamer, CH Tool & Die a sizer for the Starline brass. Pushed at 1,900 fps, a 300-grain bullet brings 2,400 foot-pounds of energy out of the muzzle. 

I once shot a walnut-size group at 200 yards with a Wilson Combat rifle in .458 SOCOM. “Looks good,” said Bill Wilson, regarding the knot with mild interest. “We expect that.” After chasing feral hogs through Texas brush, I came to appreciate not only the punch of the SOCOM, but the short barrel and fine balance of Wilson’s AR. His Kurt Buchert-designed 7.62x40mm – a .223 necked to .30 – shot flatter, killed as surely, with less recoil. A 110-grain TTSX from the 1-in-12 buttoned rifling in a Wilson barrel drew a firehose of blood from a double-lunged hog. 

Wilson Combat ARs, introduced in 1999, are beautifully machined and cycle with Swiss-watch precision. The newest cartridge from that shop, the .300 Ham’r, has a case .04 longer than the 7.62x40mm’s, but boasts a whopping .26 length advantage over the .300 Blackout (BLK). It has more powder space than either and sends 125- to 135-grain hunting bullets significantly faster, at a working pressure of 55,000 psi. The modest .309 free-bore in Wilson’s rifles keeps groups tight. New cartridges sized for the AR-15 mechanism are still spilling onto the market. Some, such as the .257 Bartz and Hornady’s 6mm ARC, offer more than a one-way ticket for coyotes. An interesting load in a carriage-class AR might leave your bolt rifles gathering a trace of dust.