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View Full Version : Gas system length effect on velocity?


Timber Wolf
February 07, 2017, 20:53
I expect barrel length to have an effect on velocity but how about gas system length? I have a suspicion based on shooting at longer ranges last week (300-400 yards). My 18" rifle-length gas AR appeared a lot flatter shooting than the 16" carbine length gas guns other guys were using.

Does the rifle length gas system bleeding gas off later boost velocity over the shorter gas systems?

hueyville
February 07, 2017, 23:04
In "The complete guide to AR 15 Accuracy" the author chronographed loads using a barrel with gas port compete open and then completely blocked. Velocity difference was not enough to be outside of standard deviation of loads. Oddly the string with gas totally open ran a little faster but barrel/ambient temperature could have been higher. Don't remember if temperatures were recorded. Here is a set of test strings on subject.

https://public.boxcloud.com/d/1/gHhjq_pQNOEbEhOpoxKFgLXwDRxkzJohlY08kUri1a8GKwB1Na 1xSlUBhfE2oT2hOVgVrC-fjZFv-dpY0mJMmKpBJd-cTq7gnb76Xhh9ZnjsnRlt-okJdLuS-5S7K2MTyPhLkSk-PgG4fnIoqQRLJzhffFwjqstvoXPZKenLlZoozTQB6yhfjXlyFD bz8duilE7oRN6JLWN0GIlzPyDYC2qLMTr39waNHmJtkfkO6wR-d3xVtPMNuoWWyfQ5eRmiTkM_2M1kmMkqncPkYGoJ7feUYgsWIT OBugflBuwQBTZMx_vj-vuH3TSIZfp1rrG3CCTV3WGjTHCBYtn-YmNZ7MV57JaHi5e5ADwrrQ_Z52pBSfRuWZ6lt1FttGxR2yDGCK YGSKNsAWNtyX5XkZSxWZcv6J3Q5YymZJCw8Ga53G0EQv2sejT5 pw7k6xxzgu10kc5saqBtUvFIxCcf9NYCcj6CHMjwDpltNdeUx_ 7XD32py0pVuluGrUogT27lfOElgNzaxSkgRNFTubbyiStwt9ZI JTxuuPvfXGPxCuzEquQ8fK0JdkDXQo7uwyFLKMT55RAkXh2mwi FnQG0qWRTEz-Egg5Ichzo4pxgnkIIK99jmaFXI5lvenGNEQeOzBZJuVHsr0wkC kUtjnji5bJgugGtPd-RcMjkRZaDJxKmH1qAJSTeAv1E1MZdDvc44iBtjUp2Vk3kVtnOM 3seZ7j4fIVSOmWxHkjxSDP-xw6iFyv1fQVckY31JdI6zSABR2fKOOsllV6QtNYuXR12--kQ0AIa2oUg83fcPt2oJkTms5eeLAl2PEuz96D8jV-XM6vg09TPEqIh-2j0JdzLV25570r2iGLcENODOMAbTk9h9KBdWdyzi5oogP0SvqM fFvC9bPvekihYp04Ey91yT4yAxLV4ysGqDLase4nABMzFn_Zqd jckQpQ25oPq9o3xoCTpkKhfqtbxdB8bLc_MUYWDtd6gIl3jE0a wmUnH96LQ36Xgp8BcoRrll13kKrGky0odzpMhC8n49-s9Y2uLrSUha3-MF_enJLoLTFuq01g6V_dWMwgO2GGtOhL49wGtJiJSBmGtvFl4N Jk4E5hblEvad1yxQotLSDmkywoq2-y5T-TntcHiUcNMVxtqj_iOVEL44ReiDDTNBlcwazNRNGUDiPOI-6VUewV0b2qXhOqU./download

David Tubbs sells a device that's service rifle legal called the Carrier Weight System (CWS) that delays premature unlocking of bolt. He claims it adds velocity and accuracy. One of the reasons have always tried to slow the gas system down by using a longer gas tube and heavier buffer so bolt stays locked long as possible and slows cyclic rate. My reasoning is a slower rifle is more reliable as reduces bullet hop in magazine and reduces chance of bolt going over top of next live round.

ByronF
February 08, 2017, 05:26
My hunch is the difference is due to the barrel length, not the gas system.

Timber Wolf
February 08, 2017, 08:03
My hunch is the difference is due to the barrel length, not the gas system.

Probably so, I know I do like this rifle and the way it functions and shoots. I thought I had a decent trigger (Hyperfire EDT2) but after I finger fudged a
Geissele SSA-E trigger in another guys gun, I ordered one. He had a high dollar LWRC .308 AR with an equally high (or higher) dollar Steiner scope. I can't afford the gun/scope but I can afford the trigger.;)

hueyville
February 08, 2017, 08:31
Your 18" barrel is my favorite length barrel in a shooter. Seldom has 20" given me enough advantage over a good 18" to merit the extra weight and length. That said have one WOA 20" 1:8 Wylde that will run as many through one hole as I can hold. Usually shoot five shot groups with three in one hole and two flyers opening group in 3/8" to 1/2" range so know the rifle is more capable than me. Keep meaning to throw a high power scope on it to see if helps me close up the groups, has a 4x-16x as general purpose glass. Someday may let someone with talent shoot it or mount up a 36x fixed power, grab a lead sled and see what it will do. It has a Geissele trigger with JP spring kit.

jhend170
February 08, 2017, 15:58
Your 18" barrel is my favorite length barrel in a shooter. Seldom has 20" given me enough advantage over a good 18" to merit the extra weight and length. That said have one WOA 20" 1:8 Wylde that will run as many through one hole as I can hold. Usually shoot five shot groups with three in one hole and two flyers opening group in 3/8" to 1/2" range so know the rifle is more capable than me. Keep meaning to throw a high power scope on it to see if helps me close up the groups, has a 4x-16x as general purpose glass. Someday may let someone with talent shoot it or mount up a 36x fixed power, grab a lead sled and see what it will do. It has a Geissele trigger with JP spring kit.

If you look at a graph of velocity vs barrel length about 18.25" is where the powder burn is complete, and acceleration starts to level off. 18" to 20" gains you about 50fps, but 16" to 18" gets you an extra 200fps or so. Beyond that 20" mark (with factory ammo at least) you're carrying weight for no appreciative reason. That's why my longest AR is 18" long, just not enough to gain for the penalty.

Jaxxas
February 08, 2017, 21:25
Well I know almost nothing about almost everything, but it would be difficult to believe that a longer length gas tube wouldn't produce higher velocities than a shorter length gas tube for a given length barrel without seeing some hard data.

YMMV

ByronF
February 09, 2017, 05:01
Well I know almost nothing about almost everything, but it would be difficult to believe that a longer length gas tube wouldn't produce higher velocities than a shorter length gas tube for a given length barrel without seeing some hard data.

YMMV

For lack of no data I also agree with your disagreement that the probability of increasingly slower velocities seems less likely.

hueyville
February 09, 2017, 08:49
For lack of no data I also agree with your disagreement that the probability of increasingly slower velocities seems less likely.

Decent discussion on subject at barfcom:

http://www.ar15.com/forums/t_3_118/399401_Does_Gas_System_Length_Affect_Velocity___.h tml

jhend170
February 09, 2017, 08:57
Well I know almost nothing about almost everything, but it would be difficult to believe that a longer length gas tube wouldn't produce higher velocities than a shorter length gas tube for a given length barrel without seeing some hard data.

YMMV

I can see your train of thought, and it makes sense. However, there is another possibility, that there may even be an increase with shorter gas systems. Hear me out...

Pressure and volume are inversely proportional. As volume of a gas increases, the pressure steadily drops. In a Rifle length system you have the volume of barrel, gas tube, and the internals of the BCG itself. Compare that to the pistol system and there is less volume, mainly associated with the gas tube. So, less system volume should mean greater system pressure, IF the system stayed closed long enough for the projectile to exit the barrel.

Once the BCG disengages the gas tube all bets are off because the system is no longer closed. We all know that short gas systems are notorious for cycling too fast due to the lack of dwell time. Question is, in equal-length barrels with differing length gas systems, is the bullet gone before the system opens up? I'm thinking yes, no matter the length of the gas system, because of the huge head start the projo has. It just takes a while for the gas to make it to the port, down the tube, and into the BCG, and then unlock the bolt, then overcome its mass AND the spring to get it moving. The bullet has to be long gone by then, otherwise the movement of the BCG would consistently screw up your POI.

So, to add to the confusion, where do external piston systems come in? It would make sense they would be consistently higher as they have the lowest gas volume necessary to operate. I have the Adams Arms system on everything I own. What about the few long-stroke systems out there?

So, while I'm not saying short systems ARE faster, the possibility is certainly there. In the end though I don't think you can manufacture ammo perfectly enough to really measure and see how little of a difference there really is. With an 18" barrel though it might be possible as, those come in 3 different gas lengths.

So, ultimately, would it mean anything if you answered the question? Only that the question was answered. Otherwise this is just useless ponderings that force us to think about such things, and exercising the noodle on the regular is a good thing.

hueyville
February 09, 2017, 15:55
I am going with David Tubbs who claims longer keep bolt closed, higher and more consistent the velocity. I know he sells parts but seems to know his stuff and everything purchased from him performed to claims. Hard to say he can't shoot, just watch the videos and read the record books and match results when he shows up.

MistWolf
February 11, 2017, 18:42
The trouble with that theory is that it requires extraction before the bullet leaves the barrel. The carrier doesn't even begin to move until after the bullet clears the muzzle