The 500xs shoots 7.9 grain pellets at around 490 feet per second. This works out to 4 foot pounds of energy at the muzzle. Now at 15-20 yards this number has dropped to bellow 4 fpe leaving you short of the energy required to kill most game.
For rats, I would stick with a .177 shooting around 700-800fps to avoid over penetration. I have killed rats at 35 yards with .177 Superdomes at 790fps muzzle velocity. The 1000xs would be overkill in this case with the pellet passing through the target and traveling until it hits a solid object. Perhaps the 800x would be a good choice for you. Take some money and buy a good scope with the illuminated reticle for low light shooting and I think you would have a nice ratter.
Russ S.
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