I get a lot of questions like this and figured that rather than responding to one specific email I should put it out there for other people as well. Reader Pete writes:
Hi Nick, I live in Amarillo and really enjoyed your article on your 300 BLK build. Appreciate so much your sharing of experience so that I don’t have to go through all the same trial and error. I have 2 Colt M-16s that I’m going to convert to 300 Blackout. Had a few questions for you if you’d be so kind as to give me a bit of advice . . .
1) Barrel length? I know that’s a long discussion, but I’m primarily wanting close range tactics with a very short barrel (likely 9″). Wondering if you know if there’s a significant advantage in range with a 10.5″ (both sub and super-sonic). It’s really hard to find data on accuracy down-range with different lengths and types (prob just go with AAC as they seem well respected, but would really like to find the most accurate nitrided barrel made). Would be really nice to be “accurate” (reliably hit 12″x12″ target at 200..maybe 250 yards?) despite the short barrel.
2) would you build again with the same silencer? (AAC) or have you seen / heard of any better?
So many thanks for your article and any advice you can give.
Let’s start with barrel length.
Every scrap of experience I have, and every bit of advice I’ve ever been given (with the exception of the black magic that is barrel harmonics), points to the conclusion that barrel length and accuracy have nothing whatsoever to do with each other. Longer barrels are not automatically more accurate. The only thing that barrel length gets you is velocity — the longer the bullet is in the barrel and being pushed by the expanding gasses of burning gunpowder, the faster that projectile will be going.
There is, however, a point of diminishing returns. Once all of the powder in the case has been burned, there’s no more fuel to add to the combustion and the gasses are at their maximum energy. Adding more barrel length after that point might still increase the velocity a bit, but the added weight isn’t always worth the extra dollop of velocity.
For 5.56 NATO, that point is 20″ of barrel. For 300 AAC Blackout, that point is 9 inches of barrel. That number comes from multiple conversations with the people who designed the 300 AAC Blackout cartridge, did the initial testing, and who continue to design guns around that caliber. On the flip side SIG SAUER seems to think that 6 inches is perfect for them (their new MCX uses a 6-inch barrel).
Nine inches of barrel will give you the perfect balance of light weight and velocity with the 300 BLK cartridge. It was designed to burn in short barrels and anything more is really just excess weight. The only real question is what twist rate to go with. Both AAC and Noveske use a 1:7 twist rate, while the PWS 300 BLK upper uses a 1:8 twist. The faster twist rate is perfect for the 220 grain ballistic monsters that subsonic 300 BLK ammunition use, but for the lighter hunting rounds (like Barnes’ 110gr round) not so much. Those rounds like a slower twist, something closer to 1:10, but no one seems interested in making a barrel for that. PWS is as close as you can get right now. Even so, 1:7 isn’t terrible.
As for silencers, there’s some competition out there.
Personally, if I had to do it all over again, I would definitely not go with a 762-SDN-6. There are lots of better options that have come on to the market in the last few years, and the 762-SDN-6 just hasn’t held up to the newer competition.
These days, #1 on my list, especially for new silencer owners, is the Liberty Mystic-X. The ability to shoot full auto 300 BLK through an SBR with this can attached is impressive. But what really makes me love it is the wide variety of calibers it can be used on. Everything from .22lr to subsonic .308 Winchester, and even 5.56 NATO from a 16″ barrel. And thanks to the mounting system, you can get multiple end caps to thread it on everything from 1/2 x 28 threaded barrels to H&K 3-lug mounts with a few seconds of twisting. I’ve soured on fast attach systems — not least because the ratchet on my 762-SDN-6 gave out after three years — and direct thread is the way to go. However, the stainless steel core will eventually wear out from too much full-auto fun.
For those looking for a rifle can and not worrying about rimfire or pistol calibers, the SilencerCo Harvester is perfect. Able to suppress all rifle calibers up to and including 300 Winchester Magnum, it’s perfectly at home on the 300 BLK rifle offering about 40% sound reduction. The end caps on this rifle can be changed as well to mount it on different rifles with different thread pitches, but the stainless steel baffles aren’t great for full auto fire (they will probably melt). For Pete this isn’t ideal, but for the average person this would be a good option.
The best option is one that you’re not going to like: wait. Rumor has it that SIG SAUER is coming out with their new line of rifle silencers at SHOT Show next year, and they are currently appear to be quieter and more durable than anything else on the market. And cheaper to boot. Stay tuned, because we will have more information on them as soon as that information becomes available.
In short, 9 inches is perfect for a 300 BLK barrel. As for silencers, either wait for the SIG SAUER cans to come on the market or get yourself an AAC MG-SD. While you can, that is.