It's probably a combination of both Rob.
. If it's accurate, I wouldn't bother with the barrel clean before you put it away - you'll only change the leading from the pellets.

Oxidised pellets also won't do you any favours - you could try some lube but, they'll probably still be shite. I'd buy new pellets and, put a load down the barrel to see if the accuracy comes back.

Good luck 👍