300 blackout AR experts- explain please

[FL-NC]

My only 300 BLK is a Ruger bolt action. I am considering getting one of the uppers from PSA with a 16" barrel, but I noticed that they all use a "pistol length" gas tube. Can someone explain the purpose of this, because PSA apparently doesn't have tech people to explain questions like this. For my application, I will be suppressed, and want to fire both subs and standard velocity ammo (I know the zero will be different). Thanks.

 

[someguy2800]

When shooting subsonic loads with pistol powders the gas pressure drops very very quickly, so they put the gas port closer to the chamber to ensure there is still enough pressure to cycle the action. A carbine length gas system works fine for supersonic loads but can be questionable with subsonic loads.

 

[mcb]

Yep, a proper size gas port with a pistol length gas system on a 16-inch barrel results in a very reliable system in 300 BO. My 16-inch 300 blackout with a pistol length gas system will run with supers or subsonic ammo with or without my suppressor with equal reliability.

 

[BreechFace]

You got it covered by the above two.

 

[citizenconn]

For the .300 BLK it has to do with effective cycling (especially suppressed and subsonic together) and dwell time. To generalize, if you are shooting mostly subsonic & suppressed then a pistol length gas system will cycle the most effectively - it likely won't cycle with a carbine system but that depends on the size gas hole. If you are just shooting supersonic & non-suppressed then a carbine length system should work fine. Supersonic & suppressed - a pistol length gas system will probably cycle better than a carbine but both may work. Subsonic & non-suppressed - a pistol length gas system will be preferable but in my experience a carbine length system will work find, too.

This article explains it better than me:
https://www.fogammo.com/blogs/news/why-subsonic-300-blackout-needs-a-pistol-length-gas-system

This can be complicated by the length of your barrel and the change in dwell time. Of the dozen or so .300 BLK ARs I've put together I only have 2 with pistol length gas systems but they also has faster twist rates because the barrels were built to be used subsonic & suppressed 100% of the time. I had one with a carbine length gas system that cycled erratically so I enlarged the gas hole a bit and it fixed the problem. I don't like to mess around with changing buffer weights and springs and all that to adjust cycling.

 

[FL-NC]

Thanks for the replies, it sounds like this setup will cover what I'm trying to do.

 

[BreechFace]

A good way to look at it is that 300blk utilizes faster burning powders in which the pressure curve hits peak closer to the chamber than other cartridges. And it's pressure is lower than that of 5.56.

 

[N555]

If I got into 300 blackout I would set up an upper for subs and another one for full power ammo.

 

[Charlie98]

Silly question: Is it worth it to install an adjustable gas block... to tune the gas system between super and sub?

 

[citizenconn]

If I got into 300 blackout I would set up an upper for subs and another one for full power ammo.

Are you saying with different barrel lengths, such as under 16" for subs and 16" for supers? Otherwise the .300 BLK was designed to run both subs and supers, with or without suppressor, all in the same upper. Having an adjustable gas block can help tune the cycling but its not necessary.

That being said I have a number of .300 BLK uppers and would never tell anyone to not build as many as they desire.

 

[citizenconn]

Silly question: Is it worth it to install an adjustable gas block... to tune the gas system between super and sub?

I think it is worth it but not absolutely necessary in most instances. I have had to enlarge the gas hole on a couple of builds because they were undersized from the mfg and that fixed my cycling issues.

 

[mcb]

If I got into 300 blackout I would set up an upper for subs and another one for full power ammo.

With a pistol length gas system those too systems will be very close to, if not the same setup. That is one of the nice things about 300 BO. My 16-inch gun runs Barnes 110gr TAC-TX at nearly 2400 fps and Maker Rex 220gr at 1050 fps with equal reliability both with or without my suppressor mounted. My 9-inch gun run in a very similar manner. The only 300 BO I have that is not running both is my 5-inch gun but that gas system is shorter than pistol length and it does not make much sense to running supers in such a short gun. It's subs only.

Silly question: Is it worth it to install an adjustable gas block... to tune the gas system between super and sub?

Assuming the gas port is drilled the right diameter that should be unnecessary in nearly all cases.

ETA: Also remember that an adjustable gas block can only address a barrel that is over gassed from a gas port that is too larger in diameter. If the gas port is drilled to small an adjustable gas block cannot correct that problem.

 

[Varminterror]

Silly question: Is it worth it to install an adjustable gas block... to tune the gas system between super and sub?

I use AGB’s on all of my personal rifles, and recommend them for most of my customer builds, but ESPECIALLY for anyone wanting multiple power levels such as sub-sonic and super-sonic, and suppressed and non-suppressed fire. In this era of “overgassed rifles are reliable rifles,” it’s pretty rare to find a port which is undersized, but a quick ream job to open the flow is pretty straight forward - which then gives ultimate control to the gas block.

 

[BreechFace]

In this era of “overgassed rifles are reliable rifles,”

What's the easiest way to solve customer service returns on AR's not cycling? Overgas them.

Don't think this isn't what happens in some of these manufacturer's designs.

 

[Charlie98]

I use AGB’s on all of my personal rifles

The first AR-10 I built, in .308, I didn't put one on... that was a mistake. Super gassy. They all get them, now.

 

[wesmonster]

FWIW this is the best comparison I can offer.
I have two PSA .300bo uppers in 16" and 8". The 16" runs supers and subs just fine with a FA carrier (a bit heavier than an SA carrier) and an H2 buffer. The 8" did not cycle subs with a semi auto carrier and a std. weight buffer. Wound up running a lightweight carrier and a Sprinco yellow spring ( lighter than a std. carbine spring).
Pistol length systems in 16" are the standard for .300bo reliability with all loads. I've seen a few with carbine length systems but from reports they don't run subsonic unsuppressed reliably. And you'll probably have to build one if that's what you want.
.300bo supersonic is a much lower pressure load vs 5.56.

 

[someguy2800]

If I got into 300 blackout I would set up an upper for subs and another one for full power ammo.

I kinda do that. But I have them on separate lowers. My suppressed one is a 16" side charging upper with the gas block on backwards an no gas tube. I cycle it manually. This way I can use very fast pistol powders an no action cycling noise. The other is a 14" barrel with a pinned and welded krink brake and carbine gas. I only use this for supers.

 

[N555]

I kinda do that. But I have them on separate lowers. My suppressed one is a 16" side charging upper with the gas block on backwards an no gas tube. I cycle it manually. This way I can use very fast pistol powders an no action cycling noise. The other is a 14" barrel with a pinned and welded krink brake and carbine gas. I only use this for supers.

That's about how far I would be willing go when I want it quiet.

 

[someguy2800]

That's about how far I would be willing go when I want it quiet.

If you really want it as quiet as possible get a bolt action or single shot, and load it with a faster powder like 700x or titegroup. These fast powders will not produce enough gas to cycle an AR, but they produce a very low muzzle pressure and can easily get a 220 grain up to 1000 fps. I can look it up but I think it took 5 grains of 700x to get 1000 fps with a 220 grain powder coated cast bullet. A jacketed bullet would probably take more due to friction.