I have shot a few rabbits with my .22 PCP air rifle out to about 50-60yds and so long as it is a headshot they just fall over stone dead
But it is running at 32fpe which may be a bit more than the Fenix 400, if you can put a few a shots over a chrony to find out what sort of velocity & energy level its at, I reckon it will probably be around the 15-18fpe, plenty for rabbits out to 35-40yds, I have knocked them over with my .177, 13fpe FX and the poms are shooting them with sub 12fpe air rifles ( with silencers also, but we won't go there
) The main problem you will encounter will be learning to shoot that air rifle but that's part of the fun, welcome to the darkside