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ArmaLite’s AR-10 Ultra Magnum

by Scott E. Mayer   |  September 23rd, 2010 2

Providing the platform for the .300 SAUM’s role as perhaps the ultimate .30-caliber target/tactical round.

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.

After overcoming the cartridge problem by selecting the SAUM, the next obstacle was feeding those cartridges from the magazine into the chamber. Westrom points out that this was the hardest part of the development. It’s not as easy as it looks because the larger diameter cartridge case means the cartridge stack gets squeezed in the magazine. Instead of being a nicely nested series of cartridges, the larger cases form something of a staggered column in the magazine. Also because of the larger diameter cartridge case, the bullet tips are positioned lower and more near the center of the magazine. To accommodate bullet tips that were being presented into the chamber from that lower and more central position, ArmaLite had to move the feed ramps lower and closer together; hence the different barrel extension on the Ultra.


ArmaLite modified the magazine follower it uses for the 7.62 to accommodate the larger diameter .300 SAUM cartridge. Magazine capacity is reduced to five rounds with the larger cartridge.

As for the gas system, things fell nicely into place to accommodate the magnum cartridge. In the Stoner rifle system, such as you have in the AR-10, you need to have a certain amount of energy enter the carrier group at the right time, and with the .300 SAUM, you’re dealing with higher gas pressure than from a standard cartridge. To deal with that difference, ArmaLite shifted the gas port slightly farther down the barrel to allow the gas pressure in the bore to moderate some and delay the gas pulse back to the carrier until the cartridge case shrinks away from the chamber walls.


Mechanically, a short quad rail provides that little bit of length needed for ArmaLite to use the standard gas tube on the AR-10 Ultra Mag. It also provides a popular place to mount a flashlight or a laser-sighting device.

Conveniently, moving the gas port still permitted the use of the standard gas tube by utilizing a quad rail at the front of the handguard. The rail provides that little bit of added length necessary to moderate the gas and also a place for the user to mount a flashlight or laser sighting device.

Westrom believes that the AR-10 Ultra Mag rifles will not be very forgiving in what ammunition you feed them. “It’s going to force guys to use good ammunition and proper bullets,” Westrom explained to me during an interview. Polymer-tipped hunting ammunition should work fine, as will open point or full metal-jacketed bullets. Semi-autos in general don’t like soft-point ammunition and the AR-10 Ultra promises to be no exception.


ArmaLite’s president, Mark Westrom, says the AR-10 Ultra Magnum will force shooters to use good ammunition. Bullet weights up to 180-grains were tried without any problems caused by the heavier bullets.

Bullet weight and overall length will also prove critical. Bullets weighing 150 to 168 grains shouldn’t be a problem, and Westrom was confident in using 175-grain bullets. When quizzed about 180-grain bullets, Westrom was a little less sure, but thought they should work. He answered with a blunt “I don’t know” regarding bullets weighing 190-grains, then immediately qualified his answer by pointing out that a shooter can always adjust the load and bullet seating depth for the gun.

With magnum loads naturally comes concern regarding increased wear and bore erosion. ArmaLite’s Ultras haven’t had long enough of a track record to see how much more, if any, the magnum guns wear or the bores erode than the standard caliber guns. With modern cartridge components, though, the barrel life expectancy should be good.


Semi-autos typically don’t function well with soft-point ammunition and the AR-10 Ultra promises to be no exception. Polymer-tipped, open-point and full-metal-jacketed bullets will provide the most reliable functioning.

As for accuracy, the AR-10 Ultra has far exceeded ArmaLite’s expectations. “They’re sub-minute guns,” Westrom told me. He also noted that when the standard, non-match A2 platforms are used with stainless .300 SAUM barrels, the guns routinely fire 1 1/4-inch groups. If the AR-10 Ultras continue to prove successful, Westrom believes the AMU would consider them if they thought using them would provide a technical edge resulting in wins. As for if the military would adopt the Ultra for use in the field, Westrom said that was “a much harder sell.”

Initially, proper feeding of an Ultra was going to be a handloading proposition. At the time of this writing, Remington offers the only factory loadings, and when I started working with the ArmaLite Ultra those loadings were all soft-points.

Partially heeding Westrom’s advice on bullets, I opted to assemble several handloads using premium target and hunting-style bullets ranging in weight from Hornady’s 155-grain A-Max to Nosler’s 180-grain AccuBond. I also used factory Core-Lokt and Core-Lokt Ultra soft-point hunting ammunition, and Nosler’s Partition—another style of soft-point—in a handload. Remington’s match bullet factory loading was not available until after my evaluation of the ArmaLite Ultra. By sticking with published overall cartridge length figures, I never ran into any feeding problems. I expected and received good accuracy from the various target bullets I tried, but was really shocked by the accuracy of the soft-point bullets. They defied everything Westom cautioned me about in bullets for the Ultra regarding function, though they couldn’t hold up to the accuracy of match bullets.


Though everything about the AR-10 Ultra Magnum goes against success with soft-point hunting bullets, Nosler’s venerable Partition still shot great groups at 100 yards.

Like many shooters, the Nosler Partition is a benchmark by which I compare many hunting bullets. I’ve always found it “accurate enough” for hunting, especially considering its terminal performance on large game, but it’s accuracy has never really caught my attention like it did from the Ultra. Wishful thinking had me substitute Nosler’s 180-grain AccuBond for the 180-grain Partition in the identical load hoping that lightning would strike twice, and that the polymer-tipped, bonded bullet would be equally accurate, but that wasn’t the case. Still, the AccuBond showed a lot of potential for load development if someone was going to hunt large game with ArmaLite’s Ultra.

Almost without exception I found the most accurate handloads were one step below the maximum powder charges listed in common reloading guides. Despite being a heavy bullet, the most accurate load was with Sierra’s 175-grain MatchKing bullet when powered to a conservative 2673 fps with a charge Hodgdon’s H4350 powder. One group measured a snug 0.25 inches, and that’s without any experimenting with overall cartridge length or precision case preparation. There were no apparent pressure problems with any loads shown here. With factory and handloads the cases initially showed an annoying tendency to shed a little sliver or brass from the head that would occasionally bind up the ejector rendering the Ultra a single-shot until the bolt was stripped and the brass slivers removed from the ejector and its spring. I discussed that problem with ArmaLite and they suggested that I polish the face of the ejector button. Doing so completely eliminated the malfunction caused by the softish brass and the potential for this problem has been corrected at the factory.


.300 REMINGTON SHORT-ACTION ULTRA
BULLET POWDER MUZZLE VELOCITY (fps) 100 YARD ACCURACY (ins)
(TYPE) (GRS.)
ARMALITE AR-10 ULTRA MAGNUM
Hornady 155-gr. A-Max H4895 54.2 2910 0.95
Hornady 168-gr. HPBT IMR-4064 54.1 2835 1.04
Nosler 150-gr. Ballistic Tip Reloader 15 57.0 3015 0.97
Nosler 180-gr. Partition H414 62.5 2921 1.32
Nosler 180-gr. AccuBond H414 62.5 2975 1.81
Sierra 168-gr. MatchKing Big Game 60.0 2925 0.82
Sierra 175-gr. MatchKing H4350 60.0 2673 0.70

Swift 165-gr. Scirocco
Varget 54.5 2834 0.71
Remington 165-gr. Core-Lokt Ultra Factory Load N/A N/A 2998 0.95
Remington 150-gr. PSP Factory Load N/A N/A 3122 1.30
NOTES: All handloads assembled in Remington brass with Remington 9 1/2M primers. Accuracy is the average of five, three-shot groups fired from a Caldwell “The Rock” rifle rest with sandbags. Velocity is the average of five shots measured 10 feet from the gun’s muzzle using a PACT MK IV Championship Timer and Chronograph.
NOTE: All load data should be used with caution. Always start with reduced loads first and make sure they are safe in each of your guns before proceeding to the high test loads listed. Since Shooting Times has no control over your choice of components, guns, or actual loadings, neither Shooting Times, InterMedia Outdoors, the author nor the various firearms and components manufacturers assume any responsibility for the use of this data.

As Westom claimed, the ArmaLite Ultra was easily capable of sub-moa accuracy with nearly all loads I tried. A little more experimenting with different powders, charges, and overall lengths and I’m confident the Ultra will shoot groups between 1/4 and 1/2 moa with almost any target bullet. Use a bullet that the gun really likes, throw in some extra effort in case preparation, and the Ultra will stand on its own against any gun on the High Power line, and do so with the ballistic advantage of a magnum cartridge and the functional advantage of a semi-auto.

Though ArmaLite is considering the Ultra a “stocked” model, it plans on manufacturing it in limited runs, and then selling out between runs. Because of the “quasi-custom” configuration, this factory-made gun should cost only a couple hundred more than a standard ArmaLite AR-10 rifle. Given the long range power, accuracy, and ballistic edge of the Remington Short-Action Ultra Mag in the ArmaLite, it’s almost certain that we will see shooting records broken with the Ultra combination. What is certain, though, is that the epitaph for the .300 Remington SAUM cartridge is a long way off and when it is written, it will probably be remembered as a target or tactical cartridge because of guns such as the ArmaLite Ultra.


  • Jason

    Thank you for writing this article Mr. Mayer. I own one of these .300 SAUMs. It took me two years (on and off) to get the blasted thing cycling properly due to the super tight tolerances, etc. These big bores like a solid break-in I guess (I'm having the same problem with a 6.5 Creedmoor). You insight into the history of the SAUM is very interesting, it's a shame that no one adopted it. Today you're lucky to find a single box of a single load of Remington ammo in any gun store. I'm a handloader so this is a non-issue for me. At the moment I'm trying to find load data for Superformance powder in the .300SAUM to no avail. It's used in the .300WSM and the .300RCM, but not the .300SAUM. Explain that one. Thanks again for a good article.

  • Jeff

    I, got one one those AR-10T .300 RSAUMs, had it scene 2007. Never had a feeding or cycling problem, with my handloads. I put JP large contour comp on it and, it shoots great.. The comp really settled it down, (recoils like a .308). I load with 168g sierra Match King with 59.3g of Big Game, have excellent results.

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