Sonuva gorram beach. My MP5K with a 249mm ZCI barrel - but this time with the PTFE tape removed and some actual hop applied - chronoed at 348, 346... 352.
Back in the bag it went and out came the backup G36C.
Only after getting home and jumping on AK2M4 did I have a "... wait a second..." moment and remembered that I'd already bought a 229mm ZCI, which is in the backup-backup M4 which was in the bag keeping the G36 company. Could have had that swapped in in a few minutes.
Sort yourself out, Rog.