Those I knew put 18 rds max. In my experience, carbine magazines tend to fail in two ways: The rounds will move forward and bullet points tend to drag on the front of the mag body. Reloads that are too long are notorious for this. When you combine the collective drag of the points on the mag body, and add the drag of the follower as it reaches full bottom position (it tends to rock sideways or front to rear) and at times the first round will load and the next in line just stays where it is, or maybe the rear or front of the round will pitch up, causing malfunctions. When I was in a match I would never load more than the needs of the string. Magazines tend to feed fine when only partially filled. Recoil and movement of parts tends to smooth out the flow of the cartridges in this condition. Also, with a new barrel I would consider taking a dremel tool and polishing the feed ramps of the barrel.