Quoted:
Are you getting light primer strikes or not feeding hte next round?

Light primer strikes are usually bolt bounce and failure to load the next round is not enough gas.

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I use a .082" port for 12.5" XM177E2 barrels.

ETA: the standard port for 16" CAR barrels is generally around .062"




ETA again: found this in the archives

For 5.56mm
Colt's gas port sizes are:
* 10.5" - .093
* 11.5" - .081
* 14.5" - .063
* 16" - .063

Quoted:
I would double check this by looking at the Mk18 thread - they were just talking about this, and suposedly the first ones were .077 and the LMT ones are like .088 - I'll have to go look at it myself, but if you want it to run right, I'd use Colt specs.

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Do you mean .77~.88 ?

.077 & .088 would be too small to run anything.

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He meant .077 since .75 would be 3/4 of an inch.

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More questions:

What ammo? PMC 55gr

Have you tried different mags? AWB state 20rd pinned to 15, not a lot to choose from around here

Lubrication? There's always time for lubrication

What clock position is your brass landing, if we're looking at the carbine from above? between 3 & 4

There is a lot of dwell time on a 16" CLGS, and they are often over-gassed as such, which can produce the same short-stroking because the gas impulse is so fast, it bleeds too quickly out the vent holes in the bolt carrier before enough inertia can be built up on the carrier itself to produce an ideal & full action stroke.

That said, a CLGS gas port under .060" seems small to me, but could be a decision made to mitigate the problems commonly associated with the 16"/CLGS dilemma.

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Gas block is properly aligned?  I'm wondering if you have a parts kit .625" gas block on a DD lightweight barrel, if the port is lining up, or if there is too much play between the FSB and the gas block journal. Tube had some play inside the FSB so I used High Temp RTV to seal it up

I would also double-check your gas key attachment to the carrier.  When they come loose, it is often so subtle, that it slips past even a focused inspection. Tried 3 separate BCG

Is it an AR15 carrier or M16 carrier?  Is there still that grind typical of a new BCG and fresh anodizing?  If so, you could polish the raceways on the BCG, and use plenty of SLIP 2000 lube. Freshly anodized by JT & a YM national match BCG

If all of that looks ok, I would go to the range with some M193 first with 2 or 3 buffer weights on-hand, and see what happens, then start checking function with progressively heavier buffers. I think the PMC is not a good test subject, I'll scout around for some Mil-Spec ammo

If all that fails, I'd start looking at opening the gas port.

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Also, if you have a spare recoil spring, try changing it. Tried 3 different springs & buffer, No Go

Yes, I know it sounds wrong, but a weak spring will allow everything to start moving back too soon, bleeding pressure early.

Quoted:
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.
For example;
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.

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.


The Colt milspec gas port sizes are:

10.5" .093"
11.5" .081"
14" .063"
16" .063"  I opened it from .059 to .063, if it does not run after the modification I will check other avenues before enlarging the port hole again.

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That barrel is JUNK, it will never work right so you might as well send it to me and I'll use it for a shift lever in my little red convertible LOL!!

But.... What diameter is the barrel at the gas port, if it's .750 it should be bigger, my chart shows.070 - .086, or .063 -078 for the .625 barrel.

One other thing I'd do is check how the area around the gas port looks, is the pattern even? If so that would indicate the FSB hole is properly aligned with the gas port.
Something else to check is the gas tube to FSB alignment. Put the tube in and pin it and then run a drill shank up the hole in the FSB to see if the hole in the tube matches. If not, run the drill bit up there and align it. The worst 605 I ever did, the gas tube did not align with the FSB hole. Drove me nuts!!!

Gotta run, my car is whining for me.:-)

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For reliability purposes I will dump the C-Products mags & go with the PMAGs.

Wow on the ammo!  Sounds like some good stuff.  I use mostly Hornady bullets on my re-loads but usually heavier.  I own and use $6 C-products mags for plinking.  I'm really not a P-mag fan but own plenty and it sounds like a good move.  Never had a problem shooting with either but personally never liked the fitment of either one and always have had to do a little work on them to meet my personal mag standards.  I want them to install easily and drop easily and feed correctly in all my AR's and I don't believe I've ever purchased either mfg mag that would work in all (10) without some additional finesse.  If ever a need an AR quickly I want to be able to grab mags and rifle/carbine and know they will work in any combination everytime.  Never had a FTF to date but I know what I like and want out of my mags.  P-mags need to be sanded a tad to drop and C-products have to be crammed in the mag well to seat quite often but this is just my experience with them.  I haven't bought mags besides retro in quite some time.  Don't really need more than a few hundred I wouldn't think.  

How about just using your tried and true favorite mags to diagnose problem?  Change only one thing at a time ammo or mag so you will know what fixed it.  Last batch of crap mags I saw were NIW Centers.  Good luck!