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Gary,
I have found in my sub-sonic AR builds that when using a fully loaded mag they were a little sensitive to magazine spring tension until everything is worn in as Spook mentioned....if the bullet stack puts too much drag on the bolt carrier it can slow the cycle down enough to cause a short stroke....an inexpensive experiment is to cut a couple coils at a time off the mag spring then try it with a fully loaded mag....or try putting only two or three rounds in the mag with a full length spring to see if it cycles fully...
I usually end up cutting two to four coils of my mags used in my sub-sonic AR's.....
kurtz
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