Ok, I cleaned and lubed the M4 and tried again. One fail to feed.
Ejected shell, cocked hammer and then click - didn’t chamber a round.
Took her home, did detailed clean including recoil spring. Lots of oil there but not gunked up. I did find the magazine extension a bit loose. Also front sight was loose, guess I’ll get to zero in with slugs again.
The WW shells seem to have plenty of power, is it possible I have a weak magazine spring? Does not seem to do this with AA loads.