Had a 25 cal avenger for a few months now and I get the best groups with jsb heavy 25 gr diablos. Tried fx hybrid slugs and the groups weren't a good plus they carry a bit to far to use in my neighborhood. If there is any downside to this gun its minimal The shot counts are about 25 to 30 pellets the way I have it tuned to 920 fos to shoot iguanas. If you tune it down to 700 fps the shot count will improve.. The way mine is set up, I need a compressor because most scuba tanks don't hold 300 Bar after a couple of fills. Again, if you use it at 3000 psi that will improve. My guess is that the plenum is small since it tunes best at heavy hammer spring and high regulator pressures. Putting in a larger plenum sounds reasonable but with such as small air tank, you may lose too much volume to really see any gain I have a Donny fl moderator and its ok for back yard but the gun feels huge and heavy to me. Gonna try a bullpup next. All in all, a fun gun and a great way to learn about tuning.