I just had to get my blaster rebuilt again because of a defect in my piston. The pin that holds one of the rings from spinning around messed and the ring moved around and hung up on one of the ports.

 

The factory casting/machine work within the Blaster's cylinder reflects its' mass production.
I hope your shop chamfered the port openings while they had it apart. The very edges of the port openings are way too sharp and can catch a ring easily.

Usually, the little pin in the ring land groove(s)
doesn't wear out unless there's some ring flutter
from excessive wear. Careful measurement of the ring end gap during assembly is a cure.

Flyin' Ryan