To keep the tightest groups, every Ev2 Barrel I have seen or used has needed to be cleaned about every 150 -200 pellets.
With out cleaning you see the odd flyer start to appear, that's if you can hold steady enough to notice such things and are using pellets that suit the barrel.
napier cleaning kit for mine, usually 2 or 3 pull throughs each time.
No need to take the stripper off, infact I would advise against this as if you put it back on slightly out, you may turn your EV2 into a Shotgun ( Eh Zak!
Simple solution is a straw from McDonalds, of which I have many lol