How to I make my airsoft gun more accurate?

I currently have a Airsoft Gi Custom (G&G) G4-A5 DMR and I am trying to make it as accurate as possible. It currently shoots about 430 fps with .20 grams and has a 509mm 6.03mm tight bore barrel. I have been using .25 gram bbs with it so far and the max accuracy I can get out of it is 180 feet. I was hoping to get atl east 200 feet out of this gun, so obviously I need to do some upgrading. So far all I have done is installed a madbull hop up and nub in the gun, which improved my groupings slightly. I also recently cracked open the gearbox and it seemed like the air seal was poor, so I ordered a bore up kit with a full cylinder. Unfortunately this is on pre-order, so I haven't received it yet. With the bore up kit the gun should shoot around 450-480 fps so I might switch to .28 gram bbs. Other than that, I can't think of anything else that could make my gun more accurate, so if anyone has any suggestions, it would be appreciated if you could help out. Thanks!