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.300 Blackout Ammo Guide: Supersonic vs Subsonic and AR Safety
This .300 Blackout ammo guide covers what the cartridge is, supersonic against subsonic loads, how it fits an AR-15, the one safety mistake every 300 BLK owner has to avoid, and what we stock. Our current .300 Blackout line is Magtech 123 grain FMJ, a supersonic range load, kept from the 50-round box to the 1,000-round case with a live count and 360-degree box spin on the product page.
What .300 Blackout is
.300 AAC Blackout, also listed as 300 BLK or 7.62x35mm, was developed by Advanced Armament Corp. and standardized by SAAMI in 2011. The goal was a .30 caliber bullet from an AR-15 that only needs a barrel change. The bolt, lower receiver and standard magazines stay the same. The case is formed from a shortened .223 case, which is why it fits the same magazines. It is also why the cartridge creates a real risk of mixing it up with 5.56 (see below).
Supersonic vs subsonic
- Supersonic loads: usually 110 to 125 grain bullets at roughly 2,200 to 2,400 feet per second. They carry energy similar to 7.62x39 and cycle almost any 300 BLK rifle, with or without a suppressor. The Magtech 123 grain load we stock is in this class.
- Subsonic loads: heavy bullets, often 190 to 220 grains, kept under the speed of sound, about 1,050 to 1,100 feet per second depending on temperature. With a suppressor they avoid the supersonic crack. Without one they are still loud. Subsonic ammo may need a specific gas setup or barrel length to cycle reliably.
Bullet weight changes point of impact a lot in this cartridge. Expect to re-zero or hold over when you switch between supersonic and subsonic ammunition.
The .300 Blackout we stock
Magtech Tactical .300 Blackout 123 grain FMJ (300BLKB) is a full metal jacket supersonic load for range practice, drills and function testing, in 50, 200, 250, 500 and 1,000-round packs. FMJ bullets are not designed to expand, so this is not a hunting load. Check the .300 Blackout category for other lines as they come into stock, including subsonic.
The 300 BLK and 5.56 mix-up
This is the important part. A .300 Blackout cartridge can chamber in a 5.56 or .223 rifle, because the case head and magazine fit are the same. If it is fired, the .30 caliber bullet is forced into a .22 caliber bore. The result is a catastrophic failure that can destroy the rifle and injure the shooter. It happens most often to people who own both and keep the ammo or magazines together.
- Mark 300 BLK magazines clearly, with colored tape, paint pen or a dedicated magazine color, and never load them with 5.56.
- Store 300 BLK and 5.56 ammunition in separate, labeled cans.
- Bring only one caliber to the bench at a time on a range day.
- Mark the upper itself near the ejection port or magazine well.
Twist rate and barrel length
Most 300 BLK barrels use a 1:7 or 1:8 twist, fast enough to stabilize heavy subsonic bullets. The cartridge was designed around short barrels. A 9-inch barrel burns most of the powder, and longer barrels add modest velocity. If you are building, pick the gas system length your barrel maker recommends for the ammunition you plan to run.
Buying, shipping and pickup
You must be 21 or older to order. Orders ship UPS Ground from Phoenix, and shipping is charged by weight. See where we ship. Phoenix-area shooters can call 480-623-4515 and pick up Monday to Friday, 9am to 5pm, with ID.
.300 Blackout questions
Can I use 5.56 magazines for 300 Blackout?
Standard AR-15 magazines generally work with .300 Blackout, which is part of the cartridge's appeal. Some shooters find heavy subsonic rounds feed better from certain magazine models. Mark any magazine you use for 300 BLK so it never gets mixed with 5.56.
Can you shoot 300 Blackout in a 5.56 rifle?
No, never. It can chamber, but firing it can destroy the rifle and injure you. A .300 Blackout rifle needs a .300 Blackout barrel. Keep the ammunition separated and the magazines marked.
Is 300 Blackout good for hunting in Arizona?
With an expanding hunting bullet at sensible ranges, many hunters use it for medium game. FMJ range ammo like our Magtech is not a hunting load. Check current Arizona Game and Fish regulations for your species and unit.
Do I need a suppressor to shoot subsonic 300 Blackout?
No. Subsonic ammunition fires normally without one. A suppressor is what makes subsonic quiet. Without one, subsonic still produces a loud muzzle report. Owning a suppressor in Arizona means complying with federal NFA rules.