Posted: 12/24/2003 7:13:29 PM EDT
Sorry for littering the board, but my last post I needed the gas port size for the 16" bbl. using 55-69 grainers, I also need to know what is mil-spec. location from breechface in inches, or what is best from the wealth of knowledge you guys have here on this board. I don't think 13" inches down the tube is going to work on a carbine/16" barrel, thanks again for your help.
Originally Posted By asltrfl:
I also need to know what is mil-spec. location from breechface in inches
You've got it backwards - the critical dimension is [b]not[/b] from the breech face to the gas port. It's actually the length of the barrel from the gas port to the muzzle that's most important.
A standard 20-inch barrel has the gas port located about 6 7/8" back from the muzzle (near as I can measure). With only 2 1/2" to 4" of barrel past the gas port, the old XM177E1/M639 shorties has reliability problems, even with the moderators on the muzzle to boost pressure. The dwell time of the gas pulse was just too short to cycle the action under all conditions. That's why the M4 barrels are basically the same dimension from the breech face to the gas port as the shorties, but just as long from the gas port to the muzzle as full-sized rifles.
Years ago in Machine Gun News, John Norrell published this list of port sizes -
14" or longer barrel = .063"
11.5" barrel = .081"
10" = .093"
The interesting thing is since then, Colt has decided to use the .093" gas port on [b]all[/b] barrels, including the 20" A2. I have a good friend who works at the [url=http://www.pica.army.mil/Public/]Picatinny Arsenal[/url] doing weapons development, and he tells me that .092 +.001 -.000 is the specified port size on all original drawings of A2 and M4 barrels that Colt has supplied to the military. (For more info check Colt Barrel Assembly DWG. No. 62743.)
So, no - I don't think that having the gas port 13" down from the breechface on a 16" barrel will work very well. My advice would be to drill it at a point the same as shorty (I measured 8" from the end of the barrel extension), use shorty handguards, and with 8 1/2" of dwell time it will function beautifully just like the old CAR-15s did. Or you could set it up like a "midlength" and still have about 7" of barrel past the gas port to make sure it cycles. But I think if you cut that down to 3" you're going to be spending more time cleaning and tweaking on your rifle than actually shooting it.
[Edited to ficks speling misteaks.]