The 400AR originated as a cartridge for wild hog hunting in Texas. A rugged, handy carbine with ample stopping power and plenty of quick followup shots was the developmental goal. The cartridge’s conceptual power requirement was to be at least 2,000 foot-pounds (ft-lbs) of big-bore knock-down power. The AR-15 was the chosen platform due to its ergonomics, proven utility and domestic ubiquity.
The new wildcat would be required to double-stack in AR-15 magazines, have rim compatibility with an off-the-shelf AR bolt, require no ramp or ejection port modifications, and produce maximum power. 40-caliber was selected because it is the largest bore compatible with a double-stacking case size.
The 2,000 ft-lbs goal is matched by the rimmed, turn-of-the-twentieth-century .401 Winchester Self-Loader (WSL). Loaded with a 200 grain bullet, the .401 WSL provides 2,037 ft-lbs at a muzzle velocity of 2,141 feet-per-second (FPS), with chamber pressure under 39,000 pounds-per-square inch (PSI). With a slightly longer case, the maximum allowable chamber pressure of the proposed AR wildcat would be around 50,000 PSI, which would enable it to exceed the goal of a ton of energy. And modern, expanding bullets would be employed as an improvement over the venerable .401 WSL, which was originally touted for dispatching game as large as moose and grizzlies with cast lead bullets.
The 7.35×51 Carcano case offers the right specifications to permit double-stacking and the use of a 7.62×39 bolt in an AR-15. The Carcano’s rim measures so closely to the AK’s that the brass doesn’t require modification beyond sizing up the neck and trimming to length.
Initially, Carcano cases were cut just below the shoulder, giving the case an overall length of 1.6-inches, but was later increased to 1.7″ to provide better neck tension and more durability at a cartridge overall length (COAL) of 2.24″. The original 1.6″ case had 0.30″ neck bearing on a 200 gr bullet, while the current 1.7″ (10×43) case provides a third more at 0.40″.
Two brands of jacketed bullets from muzzle loaders were selected. The 200 gr expanding bullets have a ballistic tip and one brand has a 0.4000″ diameter, while the other measures 0.3995″. Both have a ballistic coefficient of around 0.265, and produce very similar pressures and velocities. With rifle-thick gilding, these bullets are designed to dispatch game at muzzle velocities of 2,200 & 2,325 FPS.
A 40-cal/10mm 4140 chromoly round barrel blank with a 1:16-inch twist was turned to profile and threaded for the barrel extension. The blank’s original one-inch diameter was retained for the first four inches at the extension, with the remainder turned down to 0.75″. Gas block placement was determined following initial load tests.
A finishing chamber reamer with a rotating pilot was produced and the first chamber was cut in early August, 2014.
A strain gauge for measuring chamber pressure was installed on the outside of the barrel as close as possible to the barrel nut. A specialized program was employed to provide numeric and graphical representations of internal ballistics through a personal computer. Having no factory load as a baseline for the new cartridge, case heads and primers were also closely monitored for signs of excessive pressure.
Loaded cartridges were produced and results of the first series nearly achieved the ton of energy goal with a muzzle velocity of 2,104 FPS providing 1,965 ft-lbs. There were no visible signs of excessive pressure on primers, cases or bolt lugs, so additional, increased loads were tested. During later testing, a slightly overpressure (53,223 PSI) load emerged as the most promising at 2,322 FPS, producing 2,394 ft-lbs of muzzle energy, and was reduced for the standard load.
A 5-round, 6.5 Grendel stainless steel magazine was modified to accept 400AR cartridges. The front of the lips were cut back 0.15″ to promote feeding and shoulder guides were removed near the lips and crimped outward in the magazine’s body to permit unobstructed double-stacking of the straight-taper cartridges.
Mid- and carbine-length ports were tested with a 16.5″ barrel with a 1.6″ chamber, and carbine-length gas provided the best results. A standard load fully cycled the action and the double-stack magazine functioned without failure. An additional, 18″ barrel was produced to chamber the current 1.7″ case, which provides flawless function without feed ramp or ejection port modification, and uses a standard, carbine-length gas tube with a standard 0.75″ gas block.
In all, over 250 loads were tested through the course of development from the summer of 2014 to spring of 2015. Loads were worked up incrementally and stopped at any sign of over-pressure. There were several stuck-case events, but no ruptured primers or serious mishaps.
A final load was worked up to become a new standard for the 400AR (10×43). This first standard load pushed the 200 gr bullet to an average muzzle velocity of 2,312 FPS with 2,374 ft-lbs of muzzle energy. It has a Taylor Knock Out Factor of 26.42, which for FMJ or solid bullets exceeds a 200 gr .300 Winchester Magnum while producing less than half the recoil.
Additional velocity tests were made using 135, 155 and 180 grain pistol bullets with several powders. 135 gr pistol bullets gave the most remarkable results, producing a muzzle velocity of 3,042 FPS / 2,774 ft-lbs, which cycled the action without signs of excessive pressure. But alas, the load employs a pistol bullet constructed for neither high velocity nor rifle feeding.
Chambered in an AR-15, the 400AR surpassed the targeted energy goal by producing well over a ton of big-bore power while offering rapid, ample followup shots from a stout, handy carbine. It functions with proven AR-15 reliability, requires replacement of only barrel, bolt and magazine. And as it turned out, the 400AR produces the most muzzle energy of all double-stack AR-15 cartridges.
A recent iteration of the 400AR (10×43) AR-15 has a 16.5-inch barrel and weighs under 7-3/4 pounds. A fixed carbine buttstock, slim handguard and iron sights were installed for weight savings, and a buttpad, muzzle brake and forward grip were added to manage recoil.
The carbine is fitted with the first 400AR barrel produced (rechambered to the 1.7″ dimensions), which has a band at its second gas port, a holdover from the original gas block location tests. The barrel was threaded 5/8-24 to accept the muzzle brake, originally .308-caliber, bored out to accommodate the larger .40-caliber bullet.
The prototype 24-round magazine required removal of its 6.5 Grendel shoulder guides to accommodate the the straight-wall 400AR cartridge.
The recoil has been reduced to a manageable level in this configuration. With new full-on loads, recoil is a firm push instead of a sharp kick and allows quick followup shots.
The new load drives a 200-grain bullet to a muzzle velocity of 2,351 FPS / 2,455 ft-lbs in the 16.5″ barrel, surpassing the original load from an 18″ barrel. The improved load in an 18-inch barrel did even better, averaging 2,415 FPS, producing muzzle energy of more than one-and-a-quarter tons, 2,590 ft-lbs.
The 400AR already has the distinction of producing “the most muzzle energy of all double-stack AR-15 cartridges” and this carbine demonstrates an approach to its real potential as the ideal wild hog gun.