I think a lot hinges on how wide open the places you are shooting are. How close are things you cannot hit? If they are close, I would choose a sub 20 fpe airgun, 22 or 177. My Prod is about 18 fpe and my P35-177 is about the same. Neither will carry nearly as far as my more powerful airgun and I suspect either would dispatch even a large iguana with a well placed brain shot. If I was out in the open, I would want at least a 30 fpe 22. It will carry more which is a good thing as long as there is not things that cannot be hit within 500 yards or so. I would be confident with these guns with brain shots on the largest iguana within 50 yards. I would want to keep the "little guns" within 30 yards. If you want to take body shots on big iguanas you may need to step us to 40-50 fpe which is more easily reached in a 25 caliber. I would be surprised if any iguana could absorb an impact from my P35-25 to the vitals. It is 46-47 fpe, over 900 fps on 25.4s. I would expect the pellet to exit unless both front shoulders are in the pellet path - but that would be fine, the bone fragments from the first shoulder would maximize damage to the lungs.
