Let's keep in mind here that space in Japan -IS- limited as are the locations where the playing is available. When we played, it was in an actual city park along Futako Tamakawa (Futako River). Our playing 'field' was about 200 meters long by about 100 meters wide.
Yes, the guns there are VERY customized, and yes the games are alot like speedball. I must go against what AndrewL has said as I had seen INCREDIBLY little mil-sim in Japan. That's not saying China doesn't have any but there was virtually none occuring in the time I lived in Japan. There was one BIG game at the SDF base at the bottom of Mt. Fuji, but it was hardly mil-sim... imagine a 'big paintball game' complete with people dressed up as cartoon characters... yea, now you get the picture.