The first pack of KA midcaps i got literally took 4 months to break in.
I would do all the usual stuff, lubing the first bb then sending the rest down, taking out the spring and stretching it slightly and other various techniques 2 days before every game i went too and they still jammed during every single game.
Then one day they just stopped jamming and worked fine, awkward bastards they were. They still jam occasionally if i use anything but blasters.
My mp5 MAG midcaps didnt even need breaking in they just worked perfect from day one, My Beta project midcaps wouldn't work full stop untill i heatshrinked wrap around the follower to stop it moving around asmuch as it does stock.
It would be nice if midcaps had higher QC when made