Gas port issues?

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Mas Casa
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Gas port issues?

Post by Mas Casa » Sat Sep 18, 2021 7:52 pm

Apologies for the potentially misleading title of this post. For those more experienced, what gas port size(s) do you recommend for a 7" upper? I'm looking at the DDM4 PDW and the BCM 7" upper shooting Barnes 110gr TTSX, Speer 150gr GDSP, 150 gr FMJ and Hornady 190gr SubX. I plan to purchase a suppressor further in the future.

BCM said the 7" upper gas port size is proprietary info. 😞 Daniel Defense said the PDW gas port size is .113. (Said it's .094 for their 10.3" for those interested).

Everyone who I've heard owns a PDW raves about them (except maybe the trigger). Anyone have experience with the BCM 7"?

Thanks in advance!

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dellet
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Re: Gas port issues?

Post by dellet » Sun Sep 19, 2021 12:16 pm

Tough question to answer, partly because I’m not sure what it is. :mrgreen:

There is one answer for reliability, another for performance. Adding a suppressor changes those answers, as does bullet weight.

Rule of thumb is that if it functions with subs, it’s over gassed for supers. But that answer changes depending on powder choice if you

Gas tube length is another variable, but in barrels shorter than 10” your only choice is pistol.

Personally, .113” is way to big for anything other than something like a 10” carbine gas length barrel.

Manufacturers always over gas their gas ports. It cuts down on complaints from people who don’t understand why a 110 grain bullet using Unique as a powder won’t cycle at 500 fps.

To put that in a different perspective, a 16” carbine gassed barrel could benefit from a .115” port to cycle lighter than 175 grain subs without no a suppressor.

If it had pistol gas, .090” would be more than sufficient.

Buffer weight will confuse the matter more.

Bottom line is that the port needs to be tuned to the rest of the rifle and the load. Or you bore it out to .125”, which is max, and tell people who complain to buy an adjustable gas block when it won’t cycle supers because the bolt travels too fast.

Not sure it that helps, but might help you define the question a bit.
300 Blackout, not just for sub-sonics.

Mas Casa
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Re: Gas port issues?

Post by Mas Casa » Mon Sep 20, 2021 2:09 pm

Dellet,

Thanks. The question was essentially, "How much is too much?", considering the very little I know about how the variables you stated are at play. I'm inferring that an adjustable block, heavier buffer and low back pressure suppressor will slow the action and reduce wear.

Both those 7" uppers have pistol gas. BCM suggests using an H2 buffer, and Daniel Defense suggests their red (higher tension) spring supplied with the PDW, with supersonic loads.

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dellet
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Re: Gas port issues?

Post by dellet » Mon Sep 20, 2021 4:57 pm

It's kind of a trick question.

Supers, suppressed, you could probably get by with a .069" port or less.

Subs without a suppressor, you will need around .100" depending on bullet weight and powder.

Ammo manufacturers have really screwed things up, they come places like this and see people talking about shooting 150 grain subs. They gloss over the part where it's a highly modified system. Then they market 180 grain subs that won't cycle what should be considered a properly set up rifle. The gun manufacturers then start a race downhill to see who can cycle the lightest sub, without caring a shit about beating the crap out of the shooter or the rifle.

With handloads there all sorts of tricks to get things to work. I have 16" carbine gassed barrel that I shoot 175 grain subs without a suppressor. Carbine buffer which is normal for a carbine gas tube. Port is .098" and some of my handload supers will run so fast, that the bolt cycles faster than the magazine can feed.

So there are no hard and fast rules, other than if it cycles subs, it's over gassed for supers.

If you really want to tune your rifle, either an adjustable gas block, or a custom barrel with a port you specify are the options available. Any thing off the shelf will be over gassed to cut down on customer complaints.

The other problem is proper feeding. If you cut the gas back too far and use too light of buffer, there is not enough force and speed for the carrier to overcome a dirty chamber and close the bolt. There is a huge balance, and loss of performance on one end or the other, sometimes both, if you want to have it all.
300 Blackout, not just for sub-sonics.

Mas Casa
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Re: Gas port issues?

Post by Mas Casa » Mon Sep 20, 2021 8:25 pm

Much appreciated. At the end of the day my biggest concern is not beating up the rifle. Is Superlative's block the adjustable of choice, then?

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dellet
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Re: Gas port issues?

Post by dellet » Tue Sep 21, 2021 9:19 am

I am not personally familiar with the superlative blocks, but many people have posted positive results. I use the SLR blocks and have been happy.

Main thing to rememberer is if the rifle isn’t get beat up, neither are you. It’s not like it’s a heavy recoil round, but the more you can mitigate the rifle and it’s movement, the faster and more accurate your follow up shots will be. There really is not a need to sacrifice accuracy to gain reliability, but that’s exactly what you are doing with an over gassed rifle.

There is a learning curve, and you get it by feel. No matter what I say, I can’t experience it for you. For a long time a lot of barrels came with .125” ports, good to see that trend seems to be over.

Post some results and thoughts when you get it all together.
300 Blackout, not just for sub-sonics.

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B y r o n
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Re: Gas port issues?

Post by B y r o n » Tue Oct 19, 2021 7:22 pm

As was previously mentioned, different combinations will produce different results. My advise is to buy the parts that float your boat. Assemble the parts and then make adjustments were needed to fit your needs.

I put together a pistol with Ballistic Advantage 10.5" Premium barrel. I intended to use an Adams Arms piston kit on it but my handguard choice prevented me from using it, barrel nut was to long. After that I purchased a Bootleg adjustable bolt and Superlative Arms adjustable gas block along with tungsten weights to adjust the buffer weight if necessary. I intended to run it suppressed all the time with 220 gr bullets. Took it to the range the other day with bolt set to suppressed. Turned out I needed to set the bolt to unsuppressed to get the bolt to lock back. I haven't tested it yet with super sonic loads yet but at least know that between the bolt and adjustable gas block I should be able to gas it back enough to function properly.

The point I'm trying to make is until you've got a complete package and tested it with the intended ammo, you won't know what will need adjusting. Build it, test it and then make adjustments.

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