with .177 i just find you have to let them present a shot rather than guns blazin. the larger calibre is better. if your re zeroed at say 20 yards people should be able to hit the head of a rat anything between 5 and 25 yards surely. also if you shoot for the head and miss the chances are you will either hit the heart/lungs or completely miss. you could get a hw100k when they are available. they look good for rats because of size and the mags are good. if you want you can put the single shot adaptor on it