Love this post, wow bunch of iguana hunters on this channel!!
My take:
Accuracy first! Try to find the best accurate pellet you can.
Try to use a pellet that will expand, it will be more forgiving if your shot move a bit
Aim behind the eye, in the eye will do too. Recently I've taken some with heart shots, for this, aim on top of the front leg but a bit towards the front, just a bit. The heart shot is tricky, some times the iguana won't even know it was shot and it can walk away, but will fall after a few meters.
If you hit the heart or the brain, caliber doesn't matter, but at distance a small caliber is more challenging for this, because power retention and accuracy.
.22 hades and Polymags were doing really well on Steve's gun. I've always used .25 but I'm thinking buying the .22 barrel kit any tune the gun down for close range. I'm trying to eliminate pass through. I'm rarely shooting pass 70 yards
Hope this helps