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Thread: Gas port location
11-05-2011, 06:47 PM #1
Gas port location
I am looking for blue prints or location of the gas port on the barrel from the breach face of the AR-15, M-16 family of weapons. Pistol carbine mid-length and rifle This is for some custom builds any detailed information you can share will be greatly appreciated.I have searched other sights and this information is no where to be found.
thank you in advance. Pat
Don't forget our Vet's this Friday.
11-06-2011, 02:41 AM #2
And Go Ogle, fed simply 'ar-15 gas port location' only turns up a million or so 'hits' - many of them right here on PM.
More than one answer, as various barrel lengths ARE covered, as are port diameters - there are even links to a graph for calculating all that w/r loadings..:
Calculating gas port location for a DI AR-15 | Firearms Designer
11-06-2011, 07:37 AM #3
I have chased down the leads you have mentioned but have not yet found accurate information The few post were for rifle length gas and the measurement was taken from the torque shoulder not the breach face.I am trying to work up blue prints or get copies of same. Thank you
11-06-2011, 08:14 AM #4
And that varmint cartridge makes for a rudely noisy - and somewhat ineffective - machine pistol round anyway. Shorter barrel, lower velocity = need for more projo mass to get kinetics to remain useful. See .50 Beowulf.
Also didn't know that torque shoulder and 'breech face' had such a willy-nilly relationship. Perhaps you meant 'bolt face'?
Is 'the world' wrong to use 'torque shoulder' as a datum/nomenclature?
Might want to 'check your premises' as to why that term is so consistently used by Eugene Stoner onward. As in where IS the 'torque shoulder'?
Nomenclature. Semantics. Standard terms.
They do matter.
11-06-2011, 10:58 AM #5
You are probably better to work from trial and error than work from numbers. Most people don't have a clue how gas operated guns work but its a lot simpler than you think. There are 2 forces at work. Kinetic, shock stored energy and and applied assistance. Most semi automatics need 15,000 to 20,000 PSI to supply the kinetic part. As the bullet pass's the gas orifice the pressure hits the piston like a hammer blow and pre-loads the entire system with kinetic energy and gets everything started moving. Then pressure continues to work on the piston in a declining scale to continue the cycle. Like 4 fat guys and one skinny guy moving a car. They all give a shove and get the car going and the skinny guy stays with the car to keep it going down the road. You can pretty much locate the hole 10 to 14 inches down the barrel on any high intensity caliber and start with a very small port and open it up one number drill size at a time until it begins to function. If the empty is kicking back behind the shooter the gun is operating to slow. If it is kicking forward its generally operating to fast. You can work from numbers but it does not always work. Different throats, bore diameters and firearm weights can all affect the outcome. More often than not you are you are confined by the mechanics or even esthetics of the gun as to where the port can be located.
11-06-2011, 12:17 PM #6
Here is some info that I pulled off the net a few years back. Sorry about the formatting, the spreadsheet wont paste well into this window. Should get you reasonably close. What thermite said about the breech/bolt face. Use .750" for that delta. No pistol info, but here you go.
Brl Length (in) Brl Dia (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in)
11.50 0.625 3.850 0.081 0.089
11.50 0.750 3.850 0.086 0.094
14.50 0.625 8.375 0.063 0.078
14.50 0.750 8.375 0.070 0.086
16.00 0.625 8.375 0.063 0.078
16.00 0.750 8.375 0.070 0.086
20.00 0.625 6.875 0.086 0.093
20.00 0.750 6.875 0.093 0.096
24.00 0.825 N/A 0.089 0.089
From the barrel torque shoulder where the barrel extension torques against, it's 6.75" for carbine, 8.75" for mid-length and 12.125" for rifle to the step at the rear of the gas block seat.
Breech face is 3/4" back from the torque shoulder.
Gas port is usually 0.275" to 0.295" forward of the rear step, depending on if there is a handguard cap in the design or not.
You should be able to figure everything else out from that...
(Info From AR15Barrels.com)
11-06-2011, 05:10 PM #7
Here's a drawing of an M16 barrel I had. Hope this helps....
NRA Life Member 1976
11-07-2011, 11:29 AM #8
Thank you all for your response.
Please do not take offense The term breach face and bolt face are in fact the same reference point in gunsmithing,and yes Eugene did use torque shoulder as a reference but he was using his blueprint spec's and not modern parts that have tolerances + or - that is why I sighted bolt/breach face to eliminate any variable.There are parts out that have been reverse engineered so poorly that gas blocks obscure the barrel port.
My main goal is to establish a standard so If you build or buy a barrel you will know that brand X gas block will line up.Lets face it we like the M-16 family I'm just trying to fill in a obscure page in its history.
I am currently building two 300blk uppers one will be a sbr I know I could port it using calculated pressure curves and custom gas tubes but wish to make them reproducible as possible.Dose any one think this would make a good thread.I welcome your response.
All NFA Rules apply to these builds.
11-07-2011, 12:20 PM #9
I'm an old demo guy.
'breach face' has to do with locating the point of application of highly nitrated hydrocarbons with the INTENT of destructive results...
'Breaching charges' to be specific...
Sufficiently destructive to be clearly classed as 'single use'.
Hopefully, your AR-15 derivatives will survive to fire at least a second round...
11-07-2011, 12:45 PM #10
Well when I went to school and for the last 30 years, back when the earth cooled ......... But I supose if you want to change it, who am I to argue.
DaveE907 liked this post
11-07-2011, 01:46 PM #11
And I think you mean 'cited' not 'sighted'.
There are parts out that have been reverse engineered so poorly that gas blocks obscure the barrel port.
My main goal is to establish a standard
If you want to do something more useful, just contribute to wider dissemination of the apparently obscure or taken for granted .750 measurement from torque shoulder to breech face.
IF that had been a constant in an extra column on so many of the published tables, you'd probabaly not have started down this path. But now that you know of it, it would be a good time to rethink your plan.
...so If you build or buy a barrel you will know that brand X gas block will line up.
And junk - already ignoring known standards - will remain junk regardless.
Where's the 'beef'?
11-07-2011, 01:46 PM #12
Thank you for the correction having experience with explosives my self I'm sure it sends a chill up your spine.
I often think of breech blocks as they have no bolt.
Please forgive my poor english skills it has always been a down failing of mine.
After I have collected samples of military sanctioned rifles and blueprints and other data I will post a update.
I do hope to contribute to this site as I am a working gunsmith with 30 years experience in guns and machine tools. Pat
Last edited by P.A.R.; 11-07-2011 at 01:50 PM. Reason: spelling
11-07-2011, 01:54 PM #13