stock barrels aren't bad all the times. Sometimes, like with a brass eagle samurai, the barrel is smaller than the balls so its really tight and ball breakage is high. I know because I owned one a long time ago.
But my m3 fuel's stock barrel which I haven't changed cuz it's a temp gun is very efficient. It's about 12 inches long and its wide enough to let paintballs come out smoothly and accurately. If the barrel is shorter than 8 inches though, u may wanna worry bout accuracy though. that's why most ppl upgrade. But if it's a longer barrel it should be ok with accuracy.
But like ur last thread, if u want a barrel for that a5, don't get something that wont match ur gun. a5s would look teh best with a scenario barrel or flatline barrel. May not perform better, but it would match a lot more