old goat

Hello, new guy from Mass. been around air guns for ages but i have been impressed by
PCPs, wow have they come a long way. I got a Hatsan QE from my neighbor who moved to an elderly complex and could not take it with him. I'm an old springer guy
but gave them up because of shoulder trouble. now i fill my tank with a scuba tank my son gave me. was hoping to get some info on a new rifle to shoot barn rats at 90 yards the QE just scares them away and my granddaughter won't use the barn as a studio until i do. I'm looking at the FX King 600 in 25 cal with a Nexus 5-20x50, would i use pellets or slugs for the best results. I would like head shots don't like suffering. any info would be greatly appreciated
Thanks the old goat.