300 Blackout does something pretty amazing that no other gun can really do as well: It slings either a 125- or 225-grain bullet at supersonic or subsonic speeds, shooting suppressed or unsuppressed without any modifications – just a simple mag change. With the right configuration, a 300 Blackout Upper can do all this in either a rifle, pistol, or SBR setup without having to change a thing.
Making that magic happen requires carefully considering the “right stuff” when it comes to buying a 300 Blackout Upper. With that in mind, let’s break down what you should be looking for:
Most 300 Blackout rifle uppers will sport a 16” barrel. If you want to accurately shoot both loads (subsonic and supersonic) you’ll need to stick with a 1:7 or 1:8 twist rate. If you’re building a dedicated long-range 300 Blackout gun that shoots mostly supersonic loads, then you’ll want an upper with a slower twist rate, like a 1:10 twist rate.
If you’re building a 300 Blackout SBR or pistol, then you’ll be sticking with likely a 7.5” or 10.5” barrel. The 10.5” barrel provides the most accuracy for length, whereas the 7.5” barrel is the most compact. This is also generally considered the minimum length you’ll need to shoot either load. If you opt for a short barrel like this, you’ll need to stick with those 1:7 or 1:8 twists. Less barrel length means less rifling for your round to stabilize, which means you’ll need a tighter twist to compensate.
There ain’t much to choose from here. You’ll be sticking with a pistol or carbine gas system. Every standard production 300 Blackout upper features one of these two gas systems because they’re the only ones that can reliably cycle subsonic and supersonic loads without beating up your gun – this applies to rifles, pistols, and SBRs.
On top of that, you’ll likely need an H2 buffer to shoot both. If that doesn’t work, a general rule of thumb is that you’ll need a buffer that weighs between 3.7 and 4.3 ounces.
Lucky for you, any 300 Blackout upper receiver is just a standard AR 15 receiver. That means you’ll be sticking with the same bolt carrier group and charging handle setup, too. Because 300 Blackout loads can vary wildly, we recommend using a chrome-lined BCG to help protect against added abuse and to ensure reliable cycling (all of our 300 Blackout uppers feature chrome-lined BCGs).
Muzzle Thread Pitch
Obviously, the 300 Blackout upper will outmatch any other rifle (or sub gun) when suppressed. That means you’ll want to invest in a muzzle that isn’t imprisoned by a permanently fixed muzzle device. You’ll also want to stick with the standard, universal 5/8 x 24 thread pitch – this is the most common muzzle thread pitch used by suppressor makers.
Sights and Optics
Because 300 Blackout functions so well at long and short ranges, we strongly recommend investing in a good set of backup iron sights that can be folded down. Most AR owners invest in these sights for their .22-caliber chambered uppers, and the same rule applies even more so with this dual-purpose cartridge. Backup AR-15 sights will allow you to easily transition from long-range shooting with an optic to close-range, quick target acquisition that isn’t constricted by magnification or parallax error.