I've never used A #5, but I've tried A #9 in the powder measure that came with my old RL-450 and it absolutely hates A #9. I don't know if they changed up the powder measure between the RL-450 and RL-550 models though, but from what you describe it sounds very similar. My Dillon measure is unusable with A #9 and also with H110/W296. The grains get up in between the slides and jam it up. I use the adapter from Dillon and my old Lyman 55 measure when reloading with those powders.