Hatsan Flash, .22 or .25 for rats and mongoose?

For rats I would say the 22. It should be plenty. I had a hydra in 25 which is basically the same thing and didn’t care for it. If you tuned it for power (mid 800’s) it was just a steady flat drop off in velocity from shot to shot. No consistency to the string whatsoever. I assume this would change if tuned down but I didn’t like that so I got rid of it. Maybe the 22 isn’t as bad shooting lighter projectiles. I know you like to support locally but man there are such better 300 dollar choices out there than the flash. Not a chance I’d own another. 
 
I had both .22 and .25 flash and flashpup, they are basically the same frame and parts, they both were quite powerful, I bought them as refurbs, all the problems were fixed by them, I liked the accuracy and power of the .25, it's powerful enough for Pig too, but for the small pests a .22 will be fine, for .22 use JSB 18's or Crosman domes or hollow points, it hates all others brands of pellets, for .25 use JSB MKll's or Benjamin .25's, both are very accurate. you will need to get #006 O-rings for the fill Valve seal, they go out often, and make sure to lube the bolt with gun oil or light grease, use Silicone oil and lube the internals via through the pump hose or filler probe, 5-10 drops every 10 fills.