I know what you mean about installing a new hop up bucking, the madbull would not fit into the chamber, ended up going with a different one.
Currently i have a:
Guarder SP90
Lonex 6.03mm Steel tight bore inner barrel
Lonex hop 50% hard bucking and nub (on the nub i hatted the fact that i was turning my adjustment wheel up to 8 or 9, so i rapped a prometheus soft nub around the hard nub, now i only turn the wheel up to about 3)
Shooting at .88 (303fps) to .91J (312fps), living in japan with the 1J law :shutup2:
Yea, i could not live without the top rail on my 97 ;)