I haven't tried adjusting it yet. I was switching between the MP5 and the M4 and I could definitely see the BB's curving with the MP5 so I adjusted the hop-up on it, but the M4 was shooting pretty flat at the same range (about 50 ft) so I didn't want to mess with it.
The MP5 hop up is really easy to adjust, but I had a really wild shot with the MP5 while shooting indoors because of that. While installing the battery, I must have pushed the hop-up lever, because I put a nice dent in the living room drywall about 3 feet higher than the target and my backstop. The wife wasn't very happy about that.
