Upgrades for KJW M700

I currently have the KJW M700 coming in the mail, I know that it is a good gun right out of the box, however, I know that there have been some issues with the inner barrel not having an o-ring around it so it "jiggles" when you shake it. i was just curious what I could do to the gun to upgrade its' accuracy and power. I was also curious what size of BB to use, I was planning on using Madbull 0.36g bb's. All opinions would be appreciated!

I don't have any suggestions for specific upgrades, but I do know that any upgrade or part that works with the Tanaka M700 works in the KJW M700 (Its a 1:1 clone).
Weight of the BB's you use depends on the kind of accuracy/range/FPS you want. I suggest .30's or higher.