shot placement. From what you say Imissalot, you are a fair shot with plenty of trigger time. I believe the old mantra is ".177 for feathers, .22 for fur" this of course was before the larger calibers made their way into air guns. But if you can put that pellet where you want it consistently, then you should have no issue with .22 or .177 for birding. I personally dig .22 caliber out of all the options. .22 is quiet, you get the higher shot count, it still has respectable weight to take something bigger than a grouse if you put it where it needs to go.
I have to giggle when I see people putting moderators on .22 air rifles, the things are so quiet anyways, but I suppose if you want absolute silence a moderator for an air gun is quiet affordable. I'm used to shooting centerfire, so when ever someone says "this air gun is so loud" I scratch my head.
All the reading I've done when it comes to the argument of calibers really comes down to the factors that: larger calibers can handle wind a little better, so if you are going distance, larger can be your friend. I know Ted, bless his name, prefers the .25 (over the .30) because of the ballistics, but he's always shooting out far. Then there is simple physics in favor of larger pellets when it comes to hunting something bigger, or even smaller. The more mass you have impacting, the less precise you need to be, same goes for velocity (Ted did a great video on velocity in regards to the European vs. American guns).
the .22 gets my vote, especially if you are getting a quality rifle like the Impact. As others have said, you do have the option with those FX rifles to swap barrels, so if you do find yourself wanting that .25, or .177 its not a whole new gun you gotta buy. The Impact also gets my vote as the gun to choose, you can easily adjust -everything- on it, so if you need lower power its not out of the question to dial it down. And as Elite said, If you're going to be handling it a lot in the truck, all the more reason to go for the one that's easier to maneuver. Those FX Wildcats are sweet tools as well.
