Gas port size for a 14.5" Colt socom barrel?

[M Sadler]

How is gas pressure inside the system and what percentage of a difference does .005 make? Im genuinely curious.

Not 0.005, 0.0005...

I'm curious too. It seems to me that if the operating system was that sensitive to five ten-thousandths, ammo would be a huge issue. That's awfully small number when dealing with propellants that have different burn characteristics that would clearly be impacted by humidity, altitude and temperature.

ETA: It would be very expensive and just plain difficult from a machining standpoint to hold five tens through machining, heat treat and hard chrome.

[pezboy]

Colt first submitted the XM4 (the M4 prototype) for testing with a .062" gas port and had reliability trouble with one of the types of ammo. They opened up the gas port to .0625 and "it was determined that this gas port diameter performed well in cold temperatures with no carbines failing due to 'lack of power.'"

So in testing, they found a small change made a noticeable difference

Anything between .0620" and .0630" is acceptable according to the M4 barrel drawing. It's about a 1% difference in diameter, 3% difference in area and volume. Edit: I was tired last night and got the percentage difference mixed up.

[MistWolf]

How is gas pressure inside the system and what percentage of a difference does .005 make? Im genuinely curious.

I am not certain. Until I came across that information in the book, I would have said it couldn't make much difference at all. But I do remember that one of our resident experts posted that even small changes in port diameter mattered. I didn't get it at the time

[WS6]

I am not certain. Until I came across that information in the book, I would have said it couldn't make much difference at all. But I do remember that one of our resident experts posted that even small changes in port diameter mattered. I didn't get it at the time

0.005 is a huge increment. It's the difference between "overgassed" and "won't cycle fully".

[Ryno12]

0.005 is a huge increment. It's the difference between "overgassed" and "won't cycle fully".

It's a typo. He meant 0.0005.

[Iraqgunz]

Except I have measured dozens and dozens of 6920 and SOCOM barrels to include my own with a Meyer Pin Gage set and have found the numbers I have quoted to be consistent across the board.

The spec for the port on an M4 barrel is .0620-.0630". I have no reason to doubt the M4A1 SOCOM barrel is any different.

[JulyAZ]

Except I have measured dozens and dozens of 6920 and SOCOM barrels to include my own with a Meyer Pin Gage set and have found the numbers I have quoted to be consistent across the board.

Since the issue of gas per sizing come up constantly perhaps a sticky should be created with the listing of proper gas port sizing should be for the major barrel lengths. Seems like a new chart is in order.

IG would you be able to make such a chart as a reference for the rest of us?

[pezboy]

Except I have measured dozens and dozens of 6920 and SOCOM barrels to include my own with a Meyer Pin Gage set and have found the numbers I have quoted to be consistent across the board.

.0620" falls within the range I stated, correct?

Your gage pins are only going to get you the hole size in increments of .0010". A .0620" gage going into a hole only means that the hole is at least .0620" (not including any tolerance in the gage pin diameter and any irregularities in the hole). On an M4 barrel the gas port can be anything between those two numbers and be acceptable. If a gas port was on the upper end of the tolerance range, a .0630" gage pin wouldn't fit. Even if the hole was exactly .0630" it is possible a .0630" gage pin still wouldn't fit.

Maybe Colt makes all of their gas ports on the lower end of the tolerance range, that doesn't change what the spec is. Even if you were measuring the gas port diameter on an optical comparator and are getting .0620", that doesn't change what the spec is.

[MistWolf]

0.005 is a huge increment. It's the difference between "overgassed" and "won't cycle fully".

It's a typo. He meant 0.0005.

I should have caught that. I was talking about the difference of .0005"

Good point, Pez. A diameter of .062 + .001 is a nominal of .0625

[M Sadler]

I should have caught that. I was talking about the difference of .0005"

Good point, Pez. A diameter of .062 + .001 is a nominal of .0625

Glad to see the five tens was interweb misinformation. Is it even necessary to try and frame 0.001 as 0.0005 a side?