Best gas system length for suppressed AR-15?
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Zerodefect
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Suppressed? 14.5" Middy. Carbine would be slightly over gassed. 16" would be slightly over gassed with a middy.
But .300blk, pistol gas, 7.5-10" with the suppressor permed under a keymod rail to hit 16" OAL, is the way to go IMO.
But .300blk, pistol gas, 7.5-10" with the suppressor permed under a keymod rail to hit 16" OAL, is the way to go IMO.
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Bartholomew Roberts
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1) What length barrel?
2) Semi-auto only or full auto also?
3) Suppressed all the time or unsuppressed firing also?
2) Semi-auto only or full auto also?
3) Suppressed all the time or unsuppressed firing also?
Bartholomew Roberts
Member
I would vote midlength since a suppressor will add a lot of dwell time and midlength is a longer gas system that still has good dwell time when unsuppressed. Rifle length on a 16" is difficult to do properly unsuppressed; but would be fine with suppressor. Carbine will also run fine suppressed and unsuppressed and there is a huge knowledge base available for that combo if you do encounter problems.
You could also do an adjustable gas block; but ARs work pretty well under a wide variety of conditions as is. Adjustable gas settings frequently end up being maladjusted.
You could also do an adjustable gas block; but ARs work pretty well under a wide variety of conditions as is. Adjustable gas settings frequently end up being maladjusted.
henschman
Member
You didn't say what barrel length. Gas system length is pretty meaningless without barrel length. Keep in mind a suppressor means alot more pressure in the gas system forty a longer time, so all else equal, a setup with a longergas system and shorter dwell time will run better suppressed. But what is a lot more important than gas system length is the size of the gas port. Many people like to have a rifle set up to run reliably unsuppressed, and then add an adjustable gas block so gas can be turned down when suppressed.
wally
Member
I run 16" carbine length with a standard buffer just fine, but I feel an H buffer keeps the guts a bit cleaner and still runs fine unsuppressed.
Sam1911
Moderator Emeritus
The time that the bullet is still in the barrel but after it has passed the gas port. In other words, how long does the pressure spike last, before the bullet escapes and the pressure drops off? ***
Adding a suppressor effectively increases that time because, while gas is escaping into the suppressor and thus pressure is falling after the bullet leaves the barrel's muzzle, the closed can is still providing quite a bit of backpressure, compared to the open air.
*** -- For example, if you had a 20" long rifle length barrel, but somehow put it together with a 7" long pistol style gas system, the dwell time would be enormous. There would be high pressure in the gas system for about 13" of barrel length before that bullet escapes.
On the other hand, the old "Dissipator" style carbines that had rifle-length gas systems but cut down short barrels sometimes didn't work reliably because the pulse of gas in the tube was just so short. Maybe only 2-3" of barrel length left to go once the bullet passed the gas port.
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kimberkid
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If your going to be shooting actual subsonic ammo, and you want "quiet" have you considered a 22LR upper?
I only suggest that because I've been down that road ... Subsonic ammo in 223 calibre = expensive ammo with no more punch than 22LR
I only suggest that because I've been down that road ... Subsonic ammo in 223 calibre = expensive ammo with no more punch than 22LR
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Then dwell time has little to do with how hard the gas hits the carrier. The time it takes the bullet to travel from a pistol gas port to the muzzle of a 10.5 inch barrel is about .1 millisecond. The time it take from a pistol gas port to a 20 inch barrel is less than .4 milliseconds. The difference is less than .3 milliseconds. .3 milliseconds is 3/10,000 of a second. The carrier doesn't even begin to move until the bullet has exited the muzzle and gas pressure has begun to drop.
View attachment 730553
The reason the extra distance of barrel from port to muzzle increases gas flow through the port is because it increases the time it takes for the gas to vent after the bullet has departed. That means there greater residual pressure, which in turn increases flow through the gas port, filling the expansion chamber to operating pressure sooner. It's the increased blow down time that increases BCG speeds, not increased dwell time
View attachment 730553
The reason the extra distance of barrel from port to muzzle increases gas flow through the port is because it increases the time it takes for the gas to vent after the bullet has departed. That means there greater residual pressure, which in turn increases flow through the gas port, filling the expansion chamber to operating pressure sooner. It's the increased blow down time that increases BCG speeds, not increased dwell time
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Sam1911
Moderator Emeritus
Exactly.
Thanks for all the replies. I'm likely going with a 16" barrel carbine length gas system if it will function well both ways.
So the effect of the heavier buffers is to slow the action cycling? Would I want a heaver buffer with a suppressor given the increased duration of higher pressure in the system?
So the effect of the heavier buffers is to slow the action cycling? Would I want a heaver buffer with a suppressor given the increased duration of higher pressure in the system?
You need to balance gas flow, reciprocating mass and spring rate. So if you have too much gas flow, the answer isn't more reciprocating mass and a heavier spring, the answer is to reduce the gas flow until it's right.
The rifle system is the base line and the rifle buffer weight falls between the H2 & H3. You want to tune your AR to run with a buffer within that weight range.
What ever barrel & gas system length you choose, make sure it has the right gas port diameter, or get one with a port that's a little to large and use an adjustable gas block
The rifle system is the base line and the rifle buffer weight falls between the H2 & H3. You want to tune your AR to run with a buffer within that weight range.
What ever barrel & gas system length you choose, make sure it has the right gas port diameter, or get one with a port that's a little to large and use an adjustable gas block
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Bartholomew Roberts
Member
To some degree the AR15 is already an automatic adjusted gas system since excess gas bleeds out the two holes on the side of the carrier and out the ejection port. That is one of the reasons the system is so flexible.
The exhaust ports in the carrier do not bleed off excess pressure, they simply vent gas after the gas has done it's work. With the exception of some leakage, gas doesn't vent from the exhaust ports until after the carrier has moved far enough for the rings to wipe past them. At about that time, the gas key also disconnects from the gas tube cutting off flow to the expansion chamber.
In the patent, Eugene Stoner does describe the gas system as self regulating and it actually does a pretty good job at doing just that. I had a Colt 20 inch that, just as it came from the factory, functioned reliably without regard to what type of ammo used and without battering the rifle to death.
However, when the system is over gassed due to an overly large gas port or suppressor use, carrier speed is increased- sometimes excessively so. Another result of over gassing is more gas being vented through the gas tube and chamber end to the discomfort of the shooter.
While the carrier does not begin moving until the bullet has cleared the muzzle, there is something to be said for delaying extraction so that more gas is allowed to vent out the muzzle before unsealing the chamber.
Set up correctly, the AR gas system runs fine but there are benefits to be had with the right kind of fine tuning, especially when switching from suppressed and unsuppressed. The AR runs best when the gas system is balanced between gas flow, reciprocating mass and spring rate
Zerodefect
member
You might be over thinking it a bit. We know that Colt carbines and BCM middy uppers are unstoppable. They use .0625-.063" gas ports. Lucky for us, that's the .mil standard. So most barrels seem to be defaulting to that. If I needed insane reliability and was sweating it, I'd just get a complete BCM upper for 5.56.
.063" with 7" of dwell time is where I'd start.
I start out with an H buffer, then a H2 or H3. You can also just buy an H3 and remove weights. I'm just setting my ejection pattern. Less weight = further forward ejection pattern because the case bounces off the deflector. I shoot lefty and right handed every session. So I run a little lighter than most with a 4 clock pattern similar to a stock 6920.
Carbine is great, but middy will be better suppressed. And middy is generally smoother, but hardly so. I'd only do a carbine 16", if I had an adjustable gas block. I do think there is a low pro adjustable gas block out there somewhere. Generally I don't want an adjustable gas block, another thing to fail. I'd rather build a middy 14.5 for suppressed use, and another AR for unsuppressed use. AR's are cheap. But noones building 5.56 AR's for suppressed use anymore.
The we have gas blocks, a pinned gas block is the way to go, but as long as we're careful with setscrew types, that's fine as well. Then there's the gas tube, some cheap tubes in the past weren't sized right and leaked around the carrier, I haven't seen that in a while.
http://www.sadefensejournal.com/wp/?p=1093
http://www.dtic.mil/dtic/tr/fulltext/u2/731218.pdf
.063" with 7" of dwell time is where I'd start.
I start out with an H buffer, then a H2 or H3. You can also just buy an H3 and remove weights. I'm just setting my ejection pattern. Less weight = further forward ejection pattern because the case bounces off the deflector. I shoot lefty and right handed every session. So I run a little lighter than most with a 4 clock pattern similar to a stock 6920.
Carbine is great, but middy will be better suppressed. And middy is generally smoother, but hardly so. I'd only do a carbine 16", if I had an adjustable gas block. I do think there is a low pro adjustable gas block out there somewhere. Generally I don't want an adjustable gas block, another thing to fail. I'd rather build a middy 14.5 for suppressed use, and another AR for unsuppressed use. AR's are cheap. But noones building 5.56 AR's for suppressed use anymore.
The we have gas blocks, a pinned gas block is the way to go, but as long as we're careful with setscrew types, that's fine as well. Then there's the gas tube, some cheap tubes in the past weren't sized right and leaked around the carrier, I haven't seen that in a while.
http://www.sadefensejournal.com/wp/?p=1093
http://www.dtic.mil/dtic/tr/fulltext/u2/731218.pdf
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IndianaBoy
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As someone who has a 16in carbine that I shoot suppressed and unsuppressed, get a mid-length.
DocUSMCRetired
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I run a mid length with adjustable gas blocks on most of my systems. A lot of non-adjustable systems end up over gassed with a suppressor on them. This can lead to a number of issues. But it is also very easy to over come. To set them is extremely simple.
Adjust it until the gas is off with the suppressor on. Then increase the flow, until it cycles reliably. Give it just a bump more gas maybe 1/8th of a turn, and your set.
I have had issues with switchblocks, since I hand load. I have a couple of loads that are essentially over gassed from what I would like on middle setting, and under gassed on light setting. So I use all turn screw systems. Once you get used to them, they are super easy to use.
Adjust it until the gas is off with the suppressor on. Then increase the flow, until it cycles reliably. Give it just a bump more gas maybe 1/8th of a turn, and your set.
I have had issues with switchblocks, since I hand load. I have a couple of loads that are essentially over gassed from what I would like on middle setting, and under gassed on light setting. So I use all turn screw systems. Once you get used to them, they are super easy to use.
epwrangler
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I am shooting a 16 inch barrel with a pistol length gas system with no problems.
SKILCZ, I have not found a good source of centralized information regarding this subject. To find it on the internet, it takes searching, but there is much bad information and out and out mythology to sort through to confuse the neophyte. Unless you know what to look for, you won't know how to sort the good from the bad which makes the task even more difficult.
Experimenting with your rifle is a good way to find out for yourself but it's easy to reach the wrong conclusions when working in a vacuum. I started collecting data on port sizes but then I started experimenting with an adjustable gas block and quit worrying about port diameters. I don't have good port data to pass on.
What I can tell you is that a rifle buffer is around 5.3 oz. Start with a buffer that is as close to that weight as possible, start with a good spring and set gas flow to match. This means you'll either need to find out what a good gas port diameter is for your combination of barrel length and gas system length and make sure your barrel is made accordingly, or start with a port that is a little over large and use an adjustable gas block. Before beginning your experiments, make sure the extractor and ejector are in top condition or you will get false indications.
When tuning with an adjustable gas block, you want to close the flow until, with only one round in the magazine, the rifle ejects the spent case but does not lock back. This is the lower end of functionality. Then, increase flow until the rifle ejects and locks back fully. Then shoot the rifle to see if the flow needs to be opened up a bit more to ensure positive function every time
Experimenting with your rifle is a good way to find out for yourself but it's easy to reach the wrong conclusions when working in a vacuum. I started collecting data on port sizes but then I started experimenting with an adjustable gas block and quit worrying about port diameters. I don't have good port data to pass on.
What I can tell you is that a rifle buffer is around 5.3 oz. Start with a buffer that is as close to that weight as possible, start with a good spring and set gas flow to match. This means you'll either need to find out what a good gas port diameter is for your combination of barrel length and gas system length and make sure your barrel is made accordingly, or start with a port that is a little over large and use an adjustable gas block. Before beginning your experiments, make sure the extractor and ejector are in top condition or you will get false indications.
When tuning with an adjustable gas block, you want to close the flow until, with only one round in the magazine, the rifle ejects the spent case but does not lock back. This is the lower end of functionality. Then, increase flow until the rifle ejects and locks back fully. Then shoot the rifle to see if the flow needs to be opened up a bit more to ensure positive function every time
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Jackal
Member
The entire conversation is sort of moot when you install a quality (detent type, such as Syrac or Odinworks) adjustable gas block....
Jackal
Member
Yup. With an adjustable gas block, you can generally use a standard carbine spring/buffer. This also lets you save a little weight.
I always dial my gas block down until the rifle short strokes with Wolf/Tula steel cased ammo (lowest pressure rounds out there). Then open it up a few clicks and run full pressure ammo. Makes for a reliable, very light shooting rifle/carbine.
I always dial my gas block down until the rifle short strokes with Wolf/Tula steel cased ammo (lowest pressure rounds out there). Then open it up a few clicks and run full pressure ammo. Makes for a reliable, very light shooting rifle/carbine.
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