ArmaLite's AR-10 has more than proven the test of time. The design principle is sound and well suited to standard cartridges. Chamber it for a short mag cartridge, though, and the rules change. Chamber pressure, timing, and physical cartridge dimensions all present the challenge ArmaLite overcame in producing the AR-10 Ultra Magnum chambered for Remington's .300 Short Action Ultra Mag (.300 SAUM) cartridge. ArmaLite's Ultra began with a request from the military to participate in an informal program to develop a magnum power semi-auto rifle that would be useful in a sniper or counter-sniper role. In response to that request, ArmaLite provided the hardware that was ultimately sent to the folks most tuned to accuracy—the Army Marksmanship Unit (AMU).

AMU's first focus in this informal project centered on Winchester's .300 WSM cartridge. That initial focus determined that the .300 WSM, while an excellent cartridge in a bolt-action rifle, was not particularly suitable for a semi-automatic rifle. Internally, the sidewalls of the .300 WSM case come straight down forming nearly a right angle where they meet with the flat bottom at the cartridge web. That right angle gives the WSM a smidgen more case capacity, but results in a web too thin to support the case in semi-auto operation. AMU found that in a semi-auto, the WSM cases exhibit some bulging ahead of the extractor groove. Remington's .300 SAUM, on the other hand, has internal case sidewalls that come down and form a distinct angle before transitioning to the flat bottom surface. That slight bit more web thickness made the difference as far as AMU was concerned, so development was refocused around the SAUM.

While the .300 WSM (l.) is an excellent cartridge for bolt-action rifles, the right angle at the junction between the cartridge's sidewall and web proved a weak link in a semi-auto. The .300 SAUM (r.) has a distinct angle at the junction, resulting in a thicker, stronger web.

ArmaLite took the lessons learned from that informal military program and applied them to the Ultra. According to ArmaLite's president, Mark Westrom, the AR-10 Ultra is not a modified or custom gun. Instead, he calls it a "quasi-custom" gun because parts such as the bolt, upper receiver, and barrel extension are all machined from the ground up as factory parts. The magazine is the same as on the standard-caliber AR-10, though the polymer follower is reshaped to accommodate the different cartridge stack of the .300 SAUM. The lower receiver remains unchanged. In essence, ArmaLite is manufacturing a special top end to mount on an existing bottom end.