[lurkingknight]

low fps/double feeds are generally symptoms of the nozzle not sealing completely with the hopup bucking and tappet plate timing. If we assume that the tappet and sector gear are within standard parameters that compare to other tappet and sector gears, it falls to the hopup and nozzle seal first.

Since you replaced the barrel, make sure the rubber and barrel are seated as far back as they can go in the hopup unit. Also if you're using mid or low caps, try not filling them full to test. Some guns/mags have issues when super filled up to capacity. Also try pushing forward or pulling backwards on the mag to see if it's seating position in the magwell. Another thing to check is the grub screw under the outer barrel that secures the inner barrel in place... make sure the barrel is seated as far back as you can get it before tightening that down.