Gas port issues?

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

Moderators: gds, bakerjw, renegade

Mas Casa
Member
Posts: 19
Joined: Mon May 25, 2020 7:54 am
Location: SC

Gas port issues?

Post by Mas Casa »

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!
User avatar
dellet
Silent But Deadly
Posts: 6967
Joined: Fri Nov 30, 2012 1:25 pm

Re: Gas port issues?

Post by dellet »

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
Member
Posts: 19
Joined: Mon May 25, 2020 7:54 am
Location: SC

Re: Gas port issues?

Post by Mas Casa »

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.
User avatar
dellet
Silent But Deadly
Posts: 6967
Joined: Fri Nov 30, 2012 1:25 pm

Re: Gas port issues?

Post by dellet »

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
Member
Posts: 19
Joined: Mon May 25, 2020 7:54 am
Location: SC

Re: Gas port issues?

Post by Mas Casa »

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?
User avatar
dellet
Silent But Deadly
Posts: 6967
Joined: Fri Nov 30, 2012 1:25 pm

Re: Gas port issues?

Post by dellet »

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.
User avatar
B y r o n
Member
Posts: 29
Joined: Sun Jan 20, 2019 6:50 pm
Location: Western Kentucky

Re: Gas port issues?

Post by B y r o n »

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.
mebgardner
New Member
Posts: 6
Joined: Wed Dec 29, 2021 11:02 am

RifleSpeed Gas Block

Post by mebgardner »

Thanks for the Add, I'm pleased to find this board and to be educated.

This is a great thread regarding the tradeoffs and parameters affecting the 300 gas operating system. It speaks to why there is little (no?) discussion in the FAQ, and also why there are no lookup tables for a gas port size for any set of OS parameters. A dedicated builder might be able to develop an algorithm that could compute an acceptable range of optimum gas port size for a set of OS inputs, but I do not see something like that becoming "open source" anytime soon (if at all).

I have a 10.5" Odin Works with pistol length gas on the bench, waiting for me to mount an SA clamp-on adjustable block. Ammo choices in subs and supers are once again becoming more easily obtained, but not nearly what they were a couple years ago. I want to shoot subs and supers, and still obtain many of the "-ilities" of a good rifle: Reliability being the highest among equals. It will likely be a range toy until I sort it out.

Anyway, it seems a good adjustable gas block goes a long way towards simple "first stage" tuning after selecting some ammo (and hoping it continues to be produced, which seems more likely these days). I write "simple first stage tuning" because of the number of possible tuning parameters talked about in this thread: Gas system length, Gas port size, BCG mass, BCG velocity, buffer mass, buffer spring tensions, stiction resistance, etc. That's just the hardware, not to mention the reloading parameter possibilities.

The 300 BLK appears to have the greatest need for tunability when compared to other AR calibers, because of the missions, driven by the broad range of bullet weights. (If you want it all).

There is a new(er) adjustable gas block that I eyeballed for weeks before deciding on the more well known SA block: RifleSpeed.com

I was looking for a Adj. gas block that offered a quick method for tuning the gas amount, and this product showed up in the M4Carbine forum discussions.

The price for it made my eyes water, and then they want more $ for their proprietary gas tube, and shipping, and taxes. So, about $250 for a gas block.

I decided that was too much for a range toy. Still, for a gun that begs for quick field adjustment of the gas system, this seems to be a promising method.
cdl
Senior Silent Operator
Posts: 113
Joined: Thu Sep 15, 2016 2:30 am

Re: Gas port issues?

Post by cdl »

Hmmm,.. Am I off topic or on? Maybe just adrift...

A 10.5" almost doesn't need a AGB. It has the dwell time and is very forgiving. You start loading light charges of fast powder in shorter barrels where you've opened the gas port for function, now that's another story. Switch to a hot supersonic round, well, it's nice to be able to tame it down. You'll get better groups too.

I just installed a RifleSpeed on my 8.3" BA barrel, pistol gas and a 7 3/16" handguard (because they just say it's 7" to mess with you.) Fits behind the flash hider on setting 12 (full open) by 0.01". Just enough room. Took a Superlative off and plan to repurpose it on a "set and forget" longer barrel. I still need to wring it out, but it's well made and looks like it belongs. With my minimum setup, it's pretty tough to get my stubby fat gnarly fingers in there with all those sharp and hot edges, but I have. Down on setting 1, no gas, overcoming the spring pressure can be challenging in that confined area, but I rarely ever go that low. The detents and clicks are positive. A 8.5" barrel or 9" would be a breeze. Major improvement over a typical AGB. All this and I just bought a dozen 6" hex wrenches from McMaster's for my other blocks. Only way to have a hex when you need one is to have one everywhere. RifleSpeed solves that. I'm pretty sure it'll be a real winner.
Danimalp25
New Member
Posts: 1
Joined: Wed May 18, 2022 5:23 pm

Re: Gas port issues?

Post by Danimalp25 »

I also got a 8.3 inch barrel from ba but i have a bird cage muzzle with a crush washer because of that i can only adjust it 7 and still cant get my bolt to lock back on an empty magazine what flash hider did you use and did you use a crush washer also what plunger did you use on the gas block its comes with .917 installed and also comes with .937 in the packaging
Post Reply

Who is online

Users browsing this forum: No registered users and 63 guests