12.5 Midlength gasport

[lysander]

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.

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.