12.5 Midlength gasport

**jstone** · 06-26-21, 23:00

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.

**454308** · 05-22-21, 20:47

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

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**mig1nc** · 05-25-21, 07:17

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.

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

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**The_Swede** · 05-26-21, 17:37

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”.

**turnburglar** · 05-26-21, 19:45

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

I'd call it my lucky 13".

**Vegas** · 05-28-21, 11:31

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.

**Spooky1** · 05-31-21, 10:24

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.