The 177 850M2 claims 650 FPS with 7gr pellets, that's 6.57 FPE. the 22 gets 8-9ish FPE, still low for a decent size godzilla. Personally I wouldn't use it, especially if they are grown iguanas. If you have 30-50 feet, I'm assuming not much space. That means without a quick dispatch, its going to run off and die somewhere else. can it be done, yes. Should it be done, that's a personal decision. Personally I have set rules i follow to gain the best success of the quary staying on my property. There are the rare occasions of them getting away sure, but i'm in high 90's percent of stuff dropping on my property.
I use CO2, but only for rats, mice, and c.