I have an M1icon that I bought at a gun show (I know) that has been rebarreled with a Criterion barrel and completely reparkerized with new wood that fits nice and tight. The first few rounds through this rifle functioned correctly. I'm using Federal's American Eagle M1 rounds. Since then it fails to feed reliably. Doing a search here yeilded a cracked gas plug which I replaced. Today I fired two clips through it and the first 8 rds had to be cycled manually. On the second clip 1234 were manual, 567 fed themselves, and 8 was manual again. Here is what I know/verified.

Gas plug is good
Gas piston mic'd a little ovate or flat sided @ .525 x .523
Gas cylinder looks okay, but I can't measure the Bore where the piston rides. Exception being that the bayo lug is beat up where someone drifted it off.
The rifle passes the tilt test very easily
The man I purchased it from said he chambered the barrel very tight
Gas port is not plugged or restricted

Please help me fix my short stroke!
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