Best velocity for .22?

That’s pretty close so 500-600FPS with the CPHP should do the trick and very quiet. Test fire that into a sheet of 1/2 inch plywood and it should go through and should have no issues going through the skull of that little mongoose. At 600FPS you most likely will get pass through so I would start at 500FPS which is a lot quieter if you have a short barrel gun.
 
I've always felt 700 fps was the lowest I would want to go for .22 pellets ranging from 14.5 gr to 21 gr. 

Ideally, at any pellet speed, a "Quiet gun" usually has a decent moderator, or very efficient shroud on it. 

Are you in the U.S.? You said shooting mongoose leads me to believe you might live in South America, Africa, or perhaps Asia...???



Regards,

Kindly 'Ol Uncle Hoot
 
Peep, I think we had this conversation before. It was about your Vectis truck gun. I stand behind what I said then, 700fps is a nice place to be with a .22 shooting 14.3’s. That’s where I have my Crosman 2240 bottle gun and my Weerod tuned. I don’t like operating on the lower edge of what it takes to kill something. I prefer some wiggle room.
 
Peep, I think we had this conversation before. It was about your Vectis truck gun. I stand behind what I said then, 700fps is a nice place to be with a .22 shooting 14.3’s. That’s where I have my Crosman 2240 bottle gun and my Weerod tuned. I don’t like operating on the lower edge of what it takes to kill something. I prefer some wiggle room.




I agree that we should have some room for error. However as I learn more about air guns and some near accidents myself I’m a firm believer in operating at the safer side of what it take to get the job done. Last night I took my own advice and shot a pice of pine board at 5 yards like OP’s typical range with 14.3 crosman hollow point at 450 FPS or 5fpe and got almost half inch of penetration on pine board so more than enough room for error on rats even with a body shot. Crosman are a little harder so pure lead pellets and flat headed pellets like wade cutter would be safer at under 10 yards even if the speed goes up a little but given the beating the pine board took I highly doubt even the biggest rats will need more than 500fps with any .22 pellet to be effectively killed under 20 yards.