Subsonic build

Discussion about rifles in 300 AAC BLACKOUT (7.62x35mm), hosted by the creator of the cartridge.

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samson7x
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Location: Arkansas

Subsonic build

Post by samson7x »

I'm starting an AR build and had a few questions. It will be a 16" barrel (don't want to mess with the SBR process at this time) and I was wondering what gas system I should use. Is it illadvised to use a pistol length gas system on a 16" barrel? Or would that improve reliability? I will use subsonic ammunition with suppressor almost exclusively but I would like to be able to use supers occasionally as well. I'm guessing most of you use an adjustable gas block as well? What brands do you recommend?

Thanks for the help.
jason
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Re: Subsonic build

Post by jason »

I have a 16" PSA barrel with an AAC 762-SDN-6, with a carbine length gas system.
It cycles perfectly with suppressed subsonics and suppressed or unsuppressed supersonics. I do not have an adjustable gas block, and I don't believe you need one, unless you want to fire unsuppressed subsonics (low pressure extreme) and suppressed supersonics (high pressure extreme).

Your barrel will probably come drilled for a carbine length gas system already, so it would be a pain to make it pistol length.
bani
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Re: Subsonic build

Post by bani »

no need for adjustable gas block. my 16" PSA carbine length barrel cycles supers and subs without problems.

a 5.56 with a suppressor is really gassy. the 300blk doesn't seem to have that same problem.
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Garrett
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Re: Subsonic build

Post by Garrett »

Do you reload, or do you plan to shoot factory ammo exclusively? If you just do factory ammo, most any of the current commercially available uppers should work fine. The 300BLK was designed to work with both subsonic and supersonic ammo without having to mess with an adjustable gas block. To be honest, I'm quite impressed that the engineers at AAC / Rem were able to figure out how to do that.

Of course, there is no free lunch. The trade-off in the super/sub versatility is that you are limited in the bullet weight and powder selection for either the supersonic or subsonic loadings. If you get outside the optimal range with either, you are not guaranteed to cycle reliably.

I'd been playing with the 300 Whisper for 6 or 7 years before the 300BLK came out. This was mostly with a single-shot, where I could run pretty much any bullet / powder / OAL combo I wanted, as long as the pressure was high enough to push the bullet out the barrel, and not enough to blow up the gun.

Later, I bought a 16" Noveske 300-221 barrel with a pistol-position gas tube. This was shortly before the 300BLK came out and Noveske switched over. Noveske personnel have noted on this site that ammo loaded to 300BLK specs can safely be shot from my barrel. From what I gather, the primary difference between my 300-221 barrel, and the more recent 300BLK barrels from Noveske and most of the other current manufacturers is the size of the gas port. While I haven't bothered to measure it I'm pretty sure my gas port is going to be larger than what you will find in the current 300BLK offerings.

To me, this is a good thing because it makes it do better what I want it to. I built the 300-221 upper to shoot subsonics through. It will probably be overgassed if I shoot supers through it. But thinking back, I don't believe I've ever tried it. Like I said, that's not what I built this upper for. If I want to shoot supers, I'll shoot a different gun / caliber.

I believe RSilvers has stated that 300BLK subsonic should use 220 gr. or heavier bullets (I didn't pay attention to what the optimal supersonic bullet weights was). Also, the preferred powder seems to be A1680. While this powder runs well, it's LOUD. While it may have the optimal burn rate for subsonic 300BLK, it is not the optimal powder for suppressed shooting.

With my gun, I can reliably shoot 180 gr. bullets, and can probably go lighter. I use H110 and AA#9, both of which are quieter to my ear than A1680.

I've thought about trying to convince Noveske to do a run of Switchblocks for this barrel. But to be honest, I could probably buy a new 300BLK barrel from PSA, CMMG, or others for about the same price. With that in mind, I'll probably eventually set up a BLK upper to use primarily for supersonic ammo.
samson7x
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Posts: 123
Joined: Thu May 31, 2012 10:11 am
Location: Arkansas

Re: Subsonic build

Post by samson7x »

Garrett wrote:Do you reload, or do you plan to shoot factory ammo exclusively? If you just do factory ammo, most any of the current commercially available uppers should work fine. The 300BLK was designed to work with both subsonic and supersonic ammo without having to mess with an adjustable gas block. To be honest, I'm quite impressed that the engineers at AAC / Rem were able to figure out how to do that.

Of course, there is no free lunch. The trade-off in the super/sub versatility is that you are limited in the bullet weight and powder selection for either the supersonic or subsonic loadings. If you get outside the optimal range with either, you are not guaranteed to cycle reliably.

I'd been playing with the 300 Whisper for 6 or 7 years before the 300BLK came out. This was mostly with a single-shot, where I could run pretty much any bullet / powder / OAL combo I wanted, as long as the pressure was high enough to push the bullet out the barrel, and not enough to blow up the gun.

Later, I bought a 16" Noveske 300-221 barrel with a pistol-position gas tube. This was shortly before the 300BLK came out and Noveske switched over. Noveske personnel have noted on this site that ammo loaded to 300BLK specs can safely be shot from my barrel. From what I gather, the primary difference between my 300-221 barrel, and the more recent 300BLK barrels from Noveske and most of the other current manufacturers is the size of the gas port. While I haven't bothered to measure it I'm pretty sure my gas port is going to be larger than what you will find in the current 300BLK offerings.

To me, this is a good thing because it makes it do better what I want it to. I built the 300-221 upper to shoot subsonics through. It will probably be overgassed if I shoot supers through it. But thinking back, I don't believe I've ever tried it. Like I said, that's not what I built this upper for. If I want to shoot supers, I'll shoot a different gun / caliber.

I believe RSilvers has stated that 300BLK subsonic should use 220 gr. or heavier bullets (I didn't pay attention to what the optimal supersonic bullet weights was). Also, the preferred powder seems to be A1680. While this powder runs well, it's LOUD. While it may have the optimal burn rate for subsonic 300BLK, it is not the optimal powder for suppressed shooting.

With my gun, I can reliably shoot 180 gr. bullets, and can probably go lighter. I use H110 and AA#9, both of which are quieter to my ear than A1680.

I've thought about trying to convince Noveske to do a run of Switchblocks for this barrel. But to be honest, I could probably buy a new 300BLK barrel from PSA, CMMG, or others for about the same price. With that in mind, I'll probably eventually set up a BLK upper to use primarily for supersonic ammo.

Yes, I reload and will likely only use my reloads. That is good news that I may not need an adjustable gas block. I was thinking about getting a Core15 barrel (mainly due to price as this is a budget build) and they state that the gas port is optimized for subsonic use. It is a 1:8 barrel, but it should suit my needs just fine. Someone mentioned that using the 240gr Sierra bullets was recommended for suppressed shooting. Would H110 work well with a bullet 220gr or larger?
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