Mudder, I think you mean 5.5 FPE, not 55 FPE. Not killing a rat DRT with 5.5 FPE makes sense but not being able to do it with 55 FPE doesn't. At 55 FPE, I was thinking those are some tough rats if they're not DRT.
Are you just using body shots since they're moving? Gut shot probably wouldn't stop them DRT. Maybe a shoulder shot if head shots aren't workable? Just spitballing here. You've severely limited with the joules limitation.