The only thing I hunt with around 6fpe are rats. I've used both a Brocock Atomic and a Pardini K12, .22" and .177" respectively. I've found no difference with head shots at ranges between 3 and 20 metres; in each case it is always 1 shot, one dead rat. Rats seem to be very delicately made creatures. I'd not say the same for grey squirrels, so I go to a 20fpe rifle for those. For the past couple of months I've been staying with the match pistol Pardini on rats, right on 6fpe. That's for a couple of reasons - with the night vision and 3x scope it's a more compact package than anything else I have, and the flatter trajectory of .177" at that power level means I don't have to think about holdover between about 10 and 22 metres so it's strictly right in the crosshairs for most shots. Up close (the rats around here are often easily approached) I just remember 3cm/3m, 2cm/5m, 1cm/8m, do my range estimation then adjust between those holdovers accordingly. I really haven't found any difference between calibres for this quarry in this power range, except that the .177" pass through more reliably while the .22" more often stay in the critter. Oh, and I'm using H&N Finale Match 7.56gr in .177", JSB Exact RS 13.43gr in .22".