12.5 Midlength gasport

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05-21-21, 17:00
The_Swede

12.5 Midlength gasport

I’m building a 12.5 upper and for different reasons a chrome-lined midlength Faxxon barrel cut down from 16” to 12.5” is what I’m going to run. Gas port measures 0.075”. Gun will be used 50/50 supressed/unsupressed running M855 or similar duty grade brass cased ammo only. Gasport large enough to run it unsupressed? If not what size would you recommend? I have an adjustable gas block I could use to run different settings for suppressed/unsupressed if necessary. Any thoughts?
05-21-21, 20:55
jstone

I have a 14.5 BCM mid that im going to cut down to 12.5. I plan on leaving the gas port alone, and adjust if needed. Once you get it up and running report back here if you dont mind. I was going to send mine off last week but life decided I needed to spend money on other things. If I finish mine first I will let you know what I find.
05-22-21, 04:19
The_Swede

Quote:

Originally Posted by jstone

I have a 14.5 BCM mid that im going to cut down to 12.5. I plan on leaving the gas port alone, and adjust if needed. Once you get it up and running report back here if you dont mind. I was going to send mine off last week but life decided I needed to spend money on other things. If I finish mine first I will let you know what I find.

Sure! FYI it might take me a 2-3 weeks to get everything together and conduct a proper testfire of a few hundred rounds. Looking forward to hearing about your findings aswell.
05-22-21, 06:17
prepare

Triarc and Centurion do 12.5 mid. Don't recall the gas port size though.
05-22-21, 12:54
Clint

The 12.5" MID configuration is best suited to a dedicated suppressed role.
05-22-21, 16:23
jstone

Quote:

Originally Posted by Clint

The 12.5" MID configuration is best suited to a dedicated suppressed role.

My plan is for a dedicated suppressed rifle, but I just want to see if it will run without a suppressor. If it won't run without a suppressor I will not open the port until it runs, I may open it slightly if it appears it may run with a minor adjustment.
05-22-21, 20:04
MWAG19919

SOLGW is pretty transparent about their gas port sizes, and they do 0.078" for a 13.7 mid. They don't seem to believe in gassing guns for the bare minimum, instead preferring to design their guns to run when dirty, cold, etc. But still, I think a 0.075" 12.5" mid unsuppressed may be undergassed. According to a few sources, that's a smaller port than my 16" BCM mid (0.076").

I don't claim to know much, but I read somewhere on here that there's more to it than simple port size. Barrel profile comes into play, as well as other things like port geometry and the taper of the bore. The short version was that (all else being equal) a pencil barrel will have more gas drive than a HBAR profile barrel with the same port size.
05-22-21, 20:47
454308

I have a 12.5 mid from ar15 performance. No issues with or without suppressor and h2 buffer running 77gr mk262 clone.

Sent from my SM-G973U using Tapatalk
05-23-21, 01:50
vicious_cb

Quote:

Originally Posted by Clint

The 12.5" MID configuration is best suited to a dedicated suppressed role.

This. For both suppressed and unsuppressed get a 12.5" carbine with a 0.069" port.
05-25-21, 07:17
mig1nc

Quote:

Originally Posted by 454308

I have a 12.5 mid from ar15 performance. No issues with or without suppressor and h2 buffer running 77gr mk262 clone.

Sent from my SM-G973U using Tapatalk

IWI sells a 12.5 mid pistol Zion. If they are comfortable selling it to the general public, it must be reasonably reliable.

Sent from my iPhone using Tapatalk
05-26-21, 17:37
The_Swede

Did some measuring today in preparation of cutting. First of I measured the barrel using a rod inserted in the barrel with the barrel placed in the upper witg the BCG. Barrel-length turned out right at 16”. Measuring of that when I mark it down for 12,5” the distance between the gas port and the would-be muzzle is 2,75”. I thought the gas port on a middy was at 9” thus equaling a distance of 3,5” between gas port and the muzzle of a 12,5. Is this a potential problem? When measuring the distance from bore to gas port it shows 9,5”.
05-26-21, 19:45
turnburglar

If your custom cutting the barrel anyways... why not go with a custom size?

I'd call it my lucky 13".
05-28-21, 11:31
Vegas

I think my Noveske was approx .078 when I measured it prior to install. It was more than a minute ago. Runs well unsuppressed with brass landing around 3-4pm. With can, unsurprisingly brass lands forward around 1pm.
05-30-21, 07:09
lysander

Quote:

Originally Posted by MWAG19919

SOLGW is pretty transparent about their gas port sizes, and they do 0.078" for a 13.7 mid. They don't seem to believe in gassing guns for the bare minimum, instead preferring to design their guns to run when dirty, cold, etc. But still, I think a 0.075" 12.5" mid unsuppressed may be undergassed. According to a few sources, that's a smaller port than my 16" BCM mid (0.076").

I don't claim to know much, but I read somewhere on here that there's more to it than simple port size. Barrel profile comes into play, as well as other things like port geometry and the taper of the bore. The short version was that (all else being equal) a pencil barrel will have more gas drive than a HBAR profile barrel with the same port size.

The only part of the barrel profile that makes a difference is the diameter of the journal.

A 0.625" journal will require a reduction in port size compared to the same barrel with a 0.750" journal.
05-30-21, 09:13
prepare

https://www.m4carbine.net/showthread...rcs-12-5/page2
05-31-21, 02:22
prepare

Quote Originally Posted by MWAG19919 View Post
SOLGW is pretty transparent about their gas port sizes, and they do 0.078" for a 13.7 mid. They don't seem to believe in gassing guns for the bare minimum, instead preferring to design their guns to run when dirty, cold, etc. But still, I think a 0.075" 12.5" mid unsuppressed may be undergassed. According to a few sources, that's a smaller port than my 16" BCM mid (0.076").

I don't claim to know much, but I read somewhere on here that there's more to it than simple port size. Barrel profile comes into play, as well as other things like port geometry and the taper of the bore. The short version was that (all else being equal) a pencil barrel will have more gas drive than a HBAR profile barrel with the same port size.

Quote:

Originally Posted by lysander

The only part of the barrel profile that makes a difference is the diameter of the journal.

A 0.625" journal will require a reduction in port size compared to the same barrel with a 0.750" journal.

Can you elaborate on this?

Also is this why a H2 buffer is recommended for the HBAR barrels? Does the profile at the journal affect the timing? If so how?
05-31-21, 09:00
MWAG19919

Quote:

Originally Posted by lysander

The only part of the barrel profile that makes a difference is the diameter of the journal.

A 0.625" journal will require a reduction in port size compared to the same barrel with a 0.750" journal.

Yes that’s what I meant but you explained it better. It’s the gas journal itself that matters, but apparently it makes a difference in gas flow
05-31-21, 10:05
lysander

Quote:

Originally Posted by prepare

Quote:

Originally Posted by lysander

Quote:

Originally Posted by MWAG19919

SOLGW is pretty transparent about their gas port sizes, and they do 0.078" for a 13.7 mid. They don't seem to believe in gassing guns for the bare minimum, instead preferring to design their guns to run when dirty, cold, etc. But still, I think a 0.075" 12.5" mid unsuppressed may be undergassed. According to a few sources, that's a smaller port than my 16" BCM mid (0.076").

I don't claim to know much, but I read somewhere on here that there's more to it than simple port size. Barrel profile comes into play, as well as other things like port geometry and the taper of the bore. The short version was that (all else being equal) a pencil barrel will have more gas drive than a HBAR profile barrel with the same port size.

The only part of the barrel profile that makes a difference is the diameter of the journal.

A 0.625" journal will require a reduction in port size compared to the same barrel with a 0.750" journal.

Can you elaborate on this?

Flow through a port is governed by two things, the diameter of the hole and the length of the hole (wall friction, and boundary layers). Two port of equal diameter, but with one being longer, will see the shorter port flowing more fluid and having a smaller pressure drop. Since the port goes through the side of the barrel the thickness of the barrel at the port defines the difference.

Quote:

Originally Posted by prepare

Also is this why a H2 buffer is recommended for the HBAR barrels? Does the profile at the journal affect the timing? If so how?

In the first part of the 2000s, two changes were made to the M4A1, the buffer weight was increased, and the barrel diameter under the hand guards was increased. These two changes were completely unrelated and initially only applied to the M4A1, therefore i would guess that people assumed they were M4A1 specific. The reason these changes were not applied to the M4 was by this time the M4 was of secondary importance to the M4A1 as far as budget. It should be noted that the heavy barrel and heavy buffer have been retro-fitted to all M4s.

Personal opinion - I think the H2 buffer is a good starting place for any carbine length gas system. The H2 is closer to the original XM4 buffer design weight. However, that design did not mitigate bolt bounce vert well, and the standard carbine buffer was re-introduced on cost and schedule reasons.
05-31-21, 10:11
prepare

Quote:

Originally Posted by lysander

Flow through a port is governed by two things, the diameter of the hole and the length of the hole (wall friction, and boundary layers). Two port of equal diameter, but with one being longer, will see the shorter port flowing more fluid and having a smaller pressure drop. Since the port goes through the side of the barrel the thickness of the barrel at the port defines the difference.

In the first part of the 2000s, two changes were made to the M4A1, the buffer weight was increased, and the barrel diameter under the hand guards was increased. These two changes were completely unrelated and initially only applied to the M4A1, therefore i would guess that people assumed they were M4A1 specific. The reason these changes were not applied to the M4 was by this time the M4 was of secondary importance to the M4A1 as far as budget. It should be noted that the heavy barrel and heavy buffer have been retro-fitted to all M4s.

Personal opinion - I think the H2 buffer is a good starting place for any carbine length gas system. The H2 is closer to the original XM4 buffer design weight. However, that design did not mitigate bolt bounce vert well, and the standard carbine buffer was re-introduced on cost and schedule reasons.

Thank you again!

Was the buffer increased from a carbine buffer to a H or H to H2?
05-31-21, 10:24
Spooky1

Quote:

Originally Posted by lysander

Flow through a port is governed by two things, the diameter of the hole and the length of the hole (wall friction, and boundary layers). Two port of equal diameter, but with one being longer, will see the shorter port flowing more fluid and having a smaller pressure drop. Since the port goes through the side of the barrel the thickness of the barrel at the port defines the difference.

In the first part of the 2000s, two changes were made to the M4A1, the buffer weight was increased, and the barrel diameter under the hand guards was increased. These two changes were completely unrelated and initially only applied to the M4A1, therefore i would guess that people assumed they were M4A1 specific. The reason these changes were not applied to the M4 was by this time the M4 was of secondary importance to the M4A1 as far as budget. It should be noted that the heavy barrel and heavy buffer have been retro-fitted to all M4s.

Personal opinion - I think the H2 buffer is a good starting place for any carbine length gas system. The H2 is closer to the original XM4 buffer design weight. However, that design did not mitigate bolt bounce vert well, and the standard carbine buffer was re-introduced on cost and schedule reasons.

I also want to say thank you for sharing your knowledge.
06-01-21, 05:04
prepare

So 2 barrels of equal length, one with a 625 journal and the other with a .750- the smaller journal could or should have a smaller gat port?
06-01-21, 07:48
Clint

Correct, but the difference in port size seems to be only .001-.002", so it mostly gets ignored.

Quote:

Originally Posted by prepare

So 2 barrels of equal length, one with a 625 journal and the other with a .750- the smaller journal could or should have a smaller gas port?
06-01-21, 11:55
prepare

Quote:

Originally Posted by Clint

Correct, but the difference in port size seems to be only .001-.002", so it mostly gets ignored.

Thank you Clint.
I sent you an email about gas tubes.
06-01-21, 11:57
lysander

Quote:

Originally Posted by prepare

Thank you again!

Was the buffer increased from a carbine buffer to a H or H to H2?

The original buffer in the M4s was the standard three steel weight carbine buffer used in all Colt's shorties back to the XM177.

The new buffer is the H2 buffer, P/N 13004468.
06-02-21, 13:10
JoshNC

Quote:

Originally Posted by lysander

Flow through a port is governed by two things, the diameter of the hole and the length of the hole (wall friction, and boundary layers). Two port of equal diameter, but with one being longer, will see the shorter port flowing more fluid and having a smaller pressure drop. Since the port goes through the side of the barrel the thickness of the barrel at the port defines the difference.

In the first part of the 2000s, two changes were made to the M4A1, the buffer weight was increased, and the barrel diameter under the hand guards was increased. These two changes were completely unrelated and initially only applied to the M4A1, therefore i would guess that people assumed they were M4A1 specific. The reason these changes were not applied to the M4 was by this time the M4 was of secondary importance to the M4A1 as far as budget. It should be noted that the heavy barrel and heavy buffer have been retro-fitted to all M4s.

Personal opinion - I think the H2 buffer is a good starting place for any carbine length gas system. The H2 is closer to the original XM4 buffer design weight. However, that design did not mitigate bolt bounce vert well, and the standard carbine buffer was re-introduced on cost and schedule reasons.

Was the XM4 buffer the two piece steel design?
06-02-21, 13:53
1168

Quote:

Originally Posted by lysander

The only part of the barrel profile that makes a difference is the diameter of the journal.

A 0.625" journal will require a reduction in port size compared to the same barrel with a 0.750" journal.

Thank god someone with a grasp of physics finally posted this.
06-07-21, 03:04
The_Swede

Quote:

Originally Posted by jstone

I have a 14.5 BCM mid that im going to cut down to 12.5. I plan on leaving the gas port alone, and adjust if needed. Once you get it up and running report back here if you dont mind. I was going to send mine off last week but life decided I needed to spend money on other things. If I finish mine first I will let you know what I find.

Got the last bits mounted today and took it to the range. I have only fired 60 rounds of M855 (so take ut fir what it is) but those ran flawlessly. Gun locks back on an empty magazine on all the mags I tried (Pmag, Lancer and Troy).
06-26-21, 23:00
jstone

Quote:

Originally Posted by The_Swede

Got the last bits mounted today and took it to the range. I have only fired 60 rounds of M855 (so take ut fir what it is) but those ran flawlessly. Gun locks back on an empty magazine on all the mags I tried (Pmag, Lancer and Troy).

Thanks for the update I need to send mine out.
06-27-21, 05:06
kyjd75

Quote:

Originally Posted by prepare

Triarc and Centurion do 12.5 mid. Don't recall the gas port size though.

Looking through my past email conversations with Corrie at Centurion Arms, she indicated that they used a 0.080 port for their 12.5 midlength and 0.067 for their carbine length 12.5.
06-27-21, 15:22
w3453l

Quote:

Originally Posted by kyjd75

Looking through my past email conversations with Corrie at Centurion Arms, she indicated that they used a 0.080 port for their 12.5 midlength and 0.067 for their carbine length 12.5.

They have the gas port sizes listed on their site. As of now it looks like the 12.5 Carbine length uses a 0.064.

https://centurionarms.com/lightweigh...arrel-carbine/
06-27-21, 15:32
prepare

Quote:

Originally Posted by w3453l

They have the gas port sizes listed on their site. As of now it looks like the 12.5 Carbine length uses a 0.064.

https://centurionarms.com/lightweigh...arrel-carbine/

Thats awesome that they provide that information in the product description!
12-31-23, 16:02
JediGuy

I am bumping this thread because I have had it come up on searches a couple times.

The IWI Zion 12.5” midlength has a .085” gas port after about 3-500 rounds of use.

https://i.imgur.com/fxSLyHg.jpeg
12-31-23, 19:32
joedirt199

My homemade 12.5" mid length, 14.5 BCM cut down, has a .055 vented setscrew in the front sight base and runs 100% suppressed with a YHM turbo3. I use a standard spring and H buffer. Very smooth with very little gas to face.
03-04-24, 16:29
ErnieB

Geissele's new 12.5" CHF barrel has a gas port of .070

https://geissele.com/geissele-chf-ch...5-56-12-5.html

edited to reveal that I didn't read the title of the thread as this is a carbine length gas system barrel. :/
03-06-24, 18:40
1168

Quote:

Originally Posted by ErnieB

Geissele's new 12.5" CHF barrel has a gas port of .070

https://geissele.com/geissele-chf-ch...5-56-12-5.html

edited to reveal that I didn't read the title of the thread as this is a carbine length gas system barrel. :/

Alex, I’ll take “Whale Airway Anatomy” for $500 please. Its sad when attempts are this lazy.
03-06-24, 19:47
JediGuy

Considering my 13” will function with Wolf Gold and a .0625 gas port…yeah.