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 Post subject: AR15/M16 gas port sizes
New postPosted: 30 Apr 2006, 19:19 
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If one of the Mods would set this up as a sticky?

After doing some experimentation on various barrel lengths and gas port sizes I thought this would be a good thread for those who've done the same to post their experiences and results.

I just got a 14.5 inch DPMS M4 profile barrel from Sinistral Rifleman. This barrel is the tits and worked perfect with my full auto lower. Here's the low down.

14.5 inch barrel
gas port size .062
standard CAR gas tube
standard 3oz CAR buffer
barrel marked .223 not 5.56 (just in case chambering might make any difference in pressures)

Again, this ran perfectly, nice soft recoil, no bolt bounce or short stroking.

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New postPosted: 30 Apr 2006, 21:01 
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http://groups.msn.com/TheMarylandAR15Sh ... cefaq.msnw

Q: My Rifle has a short-stroking problem - the gunsmith says it may be a gas port problem. What are the correct sizes for the gasport?

A: Courtesy of Rick McDowell of Competition Specialties, NRA Life, USPSA Life, Master ( 105 E. Cass, PO Box 451, Osceola, Iowa 50213 Phone: (800)369-4481 / (641)342-2011 )

I have an Extraction Problem, is this related? How can they be fixed?The below gas port dimensions are the ones that I have accumulated while working on AR's for years. When someone is having problems with short stroking and their gas port is not within the below ranges, that is usually the main problem. On individual rifles I have varied both bigger and smaller with port sizes, but these were definitely exceptions, or there was no other choice due to poor quality parts we were working with.

For example;

11.5" barrel, .750" barrel diameter at the port, 3.850" from muzzle; Port should be approx. .086" to .094" in diameter. The smallest I have seen the port on these short barrels is .081" and it worked with some military ammo but not with some faster burning rate, commercial ammo. With the .086" port the military ammunition worked better but the commercial still wasn't getting enough pressure to work the carrier properly. The .094" port would work with both the high port pressure military ammo and low port pressure PMC ammo. Anything over .094" will cause short stroking/timing problems from too much pressure to the carrier and under .086" might cause short stroking for lack of pressure. To know if the port should be changed or not will depend on a few other things. Is the chamber narrower than it should be? Is the chamber rougher than it should be? Are the rings worn and allowing gas to blow by? Is the carrier key allowing gas to blow by? Etc., etc. Eliminate these variables first! With a smaller diameter barrel at the port, the gas port sizes will usually be a little smaller as well, given the barrel length is still the same.

Barrel Length (in) Barrel Diameter (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in)
11.5 .625 3.850 .081 .089
11.5 .750 3.850 .086 .094
14.5 .625 8.375 .063 .078
14.5 .750 8.375 .070 .086
16 .625 8.375 .063 .078
16 .750 8.375 .070 .086
20 .625 6.875 .086 .093
20 .750 6.875 .093 .096
24 .825 N/A .089 .089


These are of my own compiling and may not be 'spec', but I was never able to find this 'spec', so I made a list of my own from known good shooting, reliable rifles. And just as important, from the unreliable rifles that worked fine after the gas port was opened up or made smaller. Don't let a gunsmith drill out a port because of a short stroking problem that you might have! More than likely it needs to go smaller, not larger.


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 Post subject:
New postPosted: 01 May 2006, 07:25 
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I went by those figures a while back and ended up screwing up my Bushmaster 16 inch M4 barrel. I think those figures work fine for semi auto guns but when you're dealing with full auto they don't as I've found out.

Again if anyone else has done any experimenting and has some figures to post that would be great.

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New postPosted: 01 May 2006, 07:37 
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I would definitely stay on the low end of the figures knowing that the gas port is going to erode and become larger.

That is the whole theory behind drilling the gas port out past the point to which it can erode and then installing an adjustable gas tube.


Last edited by Innocent_bystander on 02 Jun 2006, 10:52, edited 1 time in total.

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 Post subject:
New postPosted: 01 May 2006, 09:57 
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Innocent_bystander wrote:
I would definitely stay on the low end of the figures knowing that the gas port is going to erode and become larger.

That is the whole theory behind drilling the gas port out past the point to which it can erode and then installing an adjustable gas tube.


That's just it, I was on the low end before I started making changes. Example, my Bushy M4 barrel was at .068 and this guys chart shows a range of .070 - .086 for that barrel. I was getting what I thought was short stroking so I opened it up to .70 and not much change, went a little more to around .072 and it got worse. So what I should have done was have the hole welded up and started even lower at around .062 which is what my DPMS barrel is at to begin with.

By doing this I was able to eliminate both the Pigtail gas tube AND the MGI heavy buffer.

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New postPosted: 20 Jul 2006, 20:08 
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I just had my 7.5" pistol filled and redrilled to .057 as I followed online info and drilled it to .093.
Ripped the heads right off the cases, then with a pigtail worked ok (didn't actuate BHO on 3 of 13 mags ran through it), but the pigtail changed colors and wound up very warped under handguards.
I'm trusting Randall at AR15barrels knows what he's doing and looking forward to trying it out.
BTW- Randall is very affordable if you need gas port work, and had mine done and back in under a week.

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New postPosted: 20 Jul 2006, 20:17 
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all dpms barrels are 556 even though they are marked 223. i dont know why but they swear it is true.

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 Post subject:
New postPosted: 20 Jul 2006, 22:43 
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chrisandclauida2 wrote:
all dpms barrels are 556 even though they are marked 223. i dont know why but they swear it is true.


Good to know. I haven't had any issues with any DPMS barrels I've owned.

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 Post subject: Re: AR15/M16 gas port sizes
New postPosted: 30 Apr 2010, 18:13 
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I had to drill my S&W MP15-T...I will go check on the size cause I dont remember :D But that damn gun would shoot more than a round or two be for it would fail to feed...


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 Post subject: Re: AR15/M16 gas port sizes
New postPosted: 11 Nov 2010, 09:19 
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Necroposting at it's finest...

Anyone know of the correct gas port size and location for a 10.5" barrel? It's in the same location as a carbine system, right?

-WRM

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 Post subject: Re: AR15/M16 gas port sizes
New postPosted: 11 Nov 2010, 10:53 
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WRMorrison wrote:
Necroposting at it's finest...

Anyone know of the correct gas port size and location for a 10.5" barrel? It's in the same location as a carbine system, right?
-WRM

93 to 96

yes

I have never needed to go 96 and have used as small as 89.

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