If you chop a 99 barrel down to 10.5 fit a wonky donky .177 drop in kit. Shoots close to the limit and a very nice quick firing cycle. Thd hw mod makes it a lot quieter too and the combination of the drop in kit and mod makes a very nice gun!
Mark williams knows of some interesting looking stuff from vorteck to bring the power up in .177. Great little guns theese hw99,s!