http://groups.msn.com/TheMarylandAR15Sh ... cefaq.msnw
Q: My Rifle has a short-stroking problem - the gunsmith says it may be a gas port problem. What are the correct sizes for the gasport?
A: Courtesy of Rick McDowell of Competition Specialties, NRA Life, USPSA Life, Master ( 105 E. Cass, PO Box 451, Osceola, Iowa 50213 Phone: (800)369-4481 / (641)342-2011 )
I have an Extraction Problem, is this related? How can they be fixed?The below gas port dimensions are the ones that I have accumulated while working on AR's for years. When someone is having problems with short stroking and their gas port is not within the below ranges, that is usually the main problem. On individual rifles I have varied both bigger and smaller with port sizes, but these were definitely exceptions, or there was no other choice due to poor quality parts we were working with.
11.5" barrel, .750" barrel diameter at the port, 3.850" from muzzle; Port should be approx. .086" to .094" in diameter. The smallest I have seen the port on these short barrels is .081" and it worked with some military ammo but not with some faster burning rate, commercial ammo. With the .086" port the military ammunition worked better but the commercial still wasn't getting enough pressure to work the carrier properly. The .094" port would work with both the high port pressure military ammo and low port pressure PMC ammo. Anything over .094" will cause short stroking/timing problems from too much pressure to the carrier and under .086" might cause short stroking for lack of pressure. To know if the port should be changed or not will depend on a few other things. Is the chamber narrower than it should be? Is the chamber rougher than it should be? Are the rings worn and allowing gas to blow by? Is the carrier key allowing gas to blow by? Etc., etc. Eliminate these variables first! With a smaller diameter barrel at the port, the gas port sizes will usually be a little smaller as well, given the barrel length is still the same.
Barrel Length (in) Barrel Diameter (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in)
11.5 .625 3.850 .081 .089
11.5 .750 3.850 .086 .094
14.5 .625 8.375 .063 .078
14.5 .750 8.375 .070 .086
16 .625 8.375 .063 .078
16 .750 8.375 .070 .086
20 .625 6.875 .086 .093
20 .750 6.875 .093 .096
24 .825 N/A .089 .089
These are of my own compiling and may not be 'spec', but I was never able to find this 'spec', so I made a list of my own from known good shooting, reliable rifles. And just as important, from the unreliable rifles that worked fine after the gas port was opened up or made smaller. Don't let a gunsmith drill out a port because of a short stroking problem that you might have! More than likely it needs to go smaller, not larger.