Noob Question about gas systems

[alamo5000]

You have rifle length, mid length, carbine and so forth.

What is the main differences and pros and cons to each?

By 'differences' I specifically mean various performance differences and when one would be used instead of the other etc.

[Slippers]

In general, the longer the gas system the better because it results in lower pressures and slower unlocking, but it has to be balanced against the dwell time. The dwell time is the time the bullet is between the gas port and muzzle, which is the only time that gas is fed down the gas tube back to the carrier. As soon as the bullet leaves the muzzle, the gas takes the path of least resistance and goes down the bore.

This is why shorter barrels have shorter gas systems, since you need to maintain some length of barrel past the gas port or risk having too short a dwell time. You can compensate for a short dwell time by increasing the gas port size, but it usually results in a smaller operating window since you're getting a brief but large volume of gas versus a more metered, constant volume of gas. This is more obvious on a 10.5" carbine gas system, because there is less dwell time compared to an 11.5" carbine length setup. In fact, this is why BCM doesn't even sell a 10.5". They're much more finicky than 11.5" barrels.

And by operating window I mean the ability for the entire system to reliably cycle different pressure ammo (5.56 vs .223, etc). Other factors enlarge the operating window like using a rifle length or A5 extension/buffer, but it helps if the barrel has the right length gas system and is ported properly from the beginning.

[alamo5000]

In general, the longer the gas system the better because it results in lower pressures and slower unlocking, but it has to be balanced against the dwell time. The dwell time is the time the bullet is between the gas port and muzzle, which is the only time that gas is fed down the gas tube back to the carrier. As soon as the bullet leaves the muzzle, the gas takes the path of least resistance and goes down the bore.

This is why shorter barrels have shorter gas systems, since you need to maintain some length of barrel past the gas port or risk having too short a dwell time. You can compensate for a short dwell time by increasing the gas port size, but it usually results in a smaller operating window since you're getting a brief but large volume of gas versus a more metered, constant volume of gas. This is more obvious on a 10.5" carbine gas system, because there is less dwell time compared to an 11.5" carbine length setup. In fact, this is why BCM doesn't even sell a 10.5". They're much more finicky than 11.5" barrels.

And by operating window I mean the ability for the entire system to reliably cycle different pressure ammo (5.56 vs .223, etc). Other factors enlarge the operating window like using a rifle length or A5 extension/buffer, but it helps if the barrel has the right length gas system and is ported properly from the beginning.

You sir are awesome. Thanks for that. Gives me plenty of info for my needs.

I found this online too.

https://www.at3tactical.com/blogs/news/9298047-carbine-mid-or-rifle-a-beginner-s-guide-to-ar15-gas-systems

[BoilerUp]

Hard to improve on Slippers' comment, but I'll ramble a bit about how I think about the AR gas system. ;)

When thinking about the "gas system", "system" is the operative word. In my mental model, you have an "action" side, and a "reaction" side. The action side are the pieces of the system that generate the forces required to cycle the bolt and include the ammunition, barrel (including gas port size and location), gas block and tube. The "reaction" side is what receives those forces: the BCG, buffer, and buffer spring. Now think of a teeter totter and you have to balance both of these on opposite sides (preferably with some wiggle room). Slippers' explained dwell time and how that factors into creating the forces on the "action" side. Shifting gas system length changes dwell time and moves that "action" side of the teeter totter back and forth (more dwell time = more force, ceteris paribus). So, when things get out of balance there are things you change on either side to bring the system back into balance (to achieve reliable function). A good factory rifle should have a "balanced" system, but the home builder may have to tinker/tune.

Note that adding a suppressor changes the "action" side, hence why AGBs are popular for those that like to switch back and forth.

However, the long dwell time that Slippers talks about also affects felt recoil. My favorite AR to shoot (in terms of felt recoil) is my 18" SPR barrel with rifle gas. That thing is just smooth, soft, and a joy to be behind. This is why most people in the know will chose a mid-length gas for a 16" barrel over a carbine gas: it's just a tad softer in felt recoil.

Here is a semi-famous post from BCM on TOS (https://www.ar15.com/forums/t_2_138/174764__Q__Why_BCM_chose_the_11_5_SBR_over_the_10_ 5_.html) on the topic:


Q: Why BCM chose the 11.5" SBR over the 10.5"

I get this question a lot. It is a good one. When we spec our program we can build anything we would like, so we sat down and looked at the pros and cons.

First Statement: I know a good 10.5" SBR can run very well. I personally own a semi MK18 type and it runs great. No problems.

A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)

If I had to "go to war" with a AR15 Carbine, I would grab the BCM 11.5".
The BCM 11.5" Runs Like a Dream.

Hope this info helps,
Paul

[Slippers]

However, the long dwell time that Slippers talks about also affects felt recoil. My favorite AR to shoot (in terms of felt recoil) is my 18" SPR barrel with rifle gas. That thing is just smooth, soft, and a joy to be behind. This is why most people in the know will chose a mid-length gas for a 16" barrel over a carbine gas: it's just a tad softer in felt recoil.


I don't think longer dwell is better as a rule. Otherwise everyone would use 20" barrels with pistol length gas. It's a balancing act.

[Aragorn]

So, how do adjustable gas blocks fit in here? For instance would there be an effective difference between reducing the gas on a carbine vs. intermediate length system? (Assume both barrels are 16")

[UWone77]

So, how do adjustable gas blocks fit in here? For instance would there be an effective difference between reducing the gas on a carbine vs. intermediate length system? (Assume both barrels are 16")

IMHO adjustable gas blocks are more for barrels that are overgassed. Shooters trying to mitigate how much freedom they get to inhale as they're shooting suppressed.

[fledge]

So, how do adjustable gas blocks fit in here? For instance would there be an effective difference between reducing the gas on a carbine vs. intermediate length system? (Assume both barrels are 16")

From my understanding, if both gas ports are tuned to a particular ammo, the longer gas system is softer shooting. However, shorter gas gives a wider range of pressure from various ammo. Thunder Ranch just released their DMR which a 20" barrel and carbine length gas. After getting flack for not running a longer gas system, Clint said, "It just works." It may mean more gas to the face if suppressed, more recoil, and more noise at the chamber, but from that perspective it means reliability (and few have seen more guns fail and succeed in class than Clint).

The adj gas block helps adjust the port to the ammo you use. An adj won't allow more gas than the port so it only helps with higher pressure ammo, which means the port is designed to operate with lesser pressured ammo. Faxon has a midlength 11.5". But they recommend an adj block because the port is large and provides more freedom to your face when suppressed. It has to be large to operate because the dwell time is short. I know it's not conventional wisdom, but I generally prefer an adj on rifles I plan to suppress regularly unless they are manufactured with a smaller port, like SIONICS 11.5. Few manufacture barrels with small ports designed for suppressors. Hopefully that will change soon.

[alamo5000]

So I think I am getting the picture here. I will try to explain in as few words as possible (but you know me). If I need corrected please do so.

1. The wild card is the ammo because rounds, powders, bullet weights, whatever can have vastly different actual PSI numbers in the chamber and consequently in the barrel.

2. Pull the trigger... then the bullet travels down the barrel. The PSI is highest at the chamber and the further the bullet travels down the barrel that PSI drops off. IE Hot gas under intense pressure progressively has more room to expand until ultimately the bullet leaves the barrel. That moment when the bullet leaves the muzzle is consequently where you get your loud 'bang' from as well. Shorter barrel=more pressure being instantly exposed to ambient temps hence a louder boom. Longer barrel gives more time for a decrease in pressure. From what I gather around 18" barrels are the optimal length for suppressors to be able to reduce sound (on a 5.56).

3. Depending on where the port is along the barrel (and it's size), as well as the actual amount of barrel length beyond the port will determine the nature of the gas being fed back through the gas tube.

For example on a really short barrel the port is physically closer to the chamber hence automatically there is much higher PSI at the port, and when the bullet de-corks after a very short dwell time this leads to a lot higher of a PSI (at the port) and a much quicker drop off in PSI after the bullet decorks. I will describe it more like a quick 'spike' that is supposed to drive your bolt etc and make the gun run.

On a longer barrel the gasses are trapped for a lot longer of a period of time after the bullet passes the port so that pressure at the tube is more like a 'pressure curve' rather than a short violent burst with an almost instant drop off. This also can be changed by physically moving the port farther out the barrel. The PSI at the chamber is one thing, 6 inches away is another, and 12 inches away is yet another and so on and so forth. The farther the port is from the chamber the actual PSI at the port becomes less.

As for adjustable blocks go, on an SBR they won't change the original nature of the pressure spike that happens at the port, but it does allow the bleeding off of some of that excess pressure before it enters the gas tube.

(EDIT: RE: Adjustable gas blocks, "As well as bleed off what would otherwise be back pressure coming backwards down the bore of the rifle")

Am I getting close?

[fledge]

As for adjustable blocks go, on an SBR they won't change the original nature of the pressure spike that happens at the port, but it does allow the bleeding off of some of that excess pressure before it enters the gas tube.

(EDIT: RE: Adjustable gas blocks, "As well as bleed off what would otherwise be back pressure coming backwards down the bore of the rifle")


Adj gas block artificially reduces port size sending more pressure toward the muzzle after the bullet passes the port. This is true if any barrel length. The bleed off system is unique to superlative and bleeds off gas after it has gone through the port once enough pressure is available to the bolt carrier (at least that's how I understand it).

Adj gas blocks in general are like A drip system in irrigation. The smaller the hole, the more water moves down the hose to the next exit point.

[Slippers]

I had a superlative and the vent settings never worked. It was like using a non-adjustable gas block that blew tons of crap onto the rear of my suppressor. The adjustment screw also snapped off and disappeared after I had used it for less than 200 rds, and the detent was very difficult to feel, making it harder to adjust.

I would only purchase an SLR Sentry.

[fledge]

I had a superlative and the vent settings never worked. It was like using a non-adjustable gas block that blew tons of crap onto the rear of my suppressor. The adjustment screw also snapped off and disappeared after I had used it for less than 200 rds, and the detent was very difficult to feel, making it harder to adjust.

I would only purchase an SLR Sentry.

Haha I reached the same conclusion. You're the only person I've heard say this publicly. I find the vent portion inconsistent. But you can use it like the sentry without the venting feature, which I do. But I won't buy another one and prefer the SLR. The SLR has firm detent and easy to rebuild if necessary.

[Aragorn]

I had a superlative and the vent settings never worked. It was like using a non-adjustable gas block that blew tons of crap onto the rear of my suppressor. The adjustment screw also snapped off and disappeared after I had used it for less than 200 rds, and the detent was very difficult to feel, making it harder to adjust.

I would only purchase an SLR Sentry.

Really? I've only ran across glowing reviews of them, and had been planning on getting one.

[Aragorn]

Haha I reached the same conclusion. You're the only person I've heard say this publicly. I find the vent portion inconsistent. But you can use it like the sentry without the venting feature, which I do. But I won't buy another one and prefer the SLR. The SLR has firm detent and easy to rebuild if necessary.

Wow you too? Alright, another SLR it'll be.

[alamo5000]

I would only purchase an SLR Sentry.

This is the block I use on both of my AR's. So far it's been great. I've set it and forgot it. It might be carbon welded by now already. LOL!

[Slippers]

Haha I reached the same conclusion. You're the only person I've heard say this publicly. I find the vent portion inconsistent. But you can use it like the sentry without the venting feature, which I do. But I won't buy another one and prefer the SLR. The SLR has firm detent and easy to rebuild if necessary.

The problem with the non-venting settings on the superlative is that they are very few. You lose over half the adjustment range.

[fledge]

It's about 4.5 rotations, as I recall. The SLR is about 7.5 at fully open to closed (though the SLR has an additional 7 to make a completely open gas block super duper completely open). The range is greater with the SLR but I generally don't tune mine to the edge of performance since I use a variety of ammo. The superlative works fine enough on that setting... until it breaks like yours.

[Default.mp3]

I would only purchase an SLR Sentry.You should buy out the MicroMOA patents and make an improved version.

[fledge]

Arisaka adjustable gas block. I'd give it a go.