12.5 Midlength gasport

[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?

[Clint]

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

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?

[prepare]

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.

[lysander]

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.

[JoshNC]

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?

[1168]

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.

[The_Swede]

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).

[jstone]

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.

[kyjd75]

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.

[w3453l]

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/