I've shot a bunch of barn pigeons with air guns, and have one with me every time I visit farms belonging to friends. I'm there to shoot groundhogs primarily, but I honestly think most farmers (here in S. Ontario) hate pigeons as much or more. lol
Pigeons are not tough birds and die easily if you do your part. I've shot lots with 22LR (CCI Quiet-22 or CB Long) in barn/basements where there are stone walls and heavy-duty floor beams above. In the barns (over hay) is where I usually shoot them. Like most people here, I have a PAL...but I use a non-PAL-rated 22 cal., Diana 24 spring piston air rifle for this. ZERO risk of punching through a tin roof. I actually don't shoot springers that well anymore, but do a little practice every spring to warm-up again. I use wad-cutter pellets to do the most damage at that lower velocity, and aim for the head if I can see it..the base of the neck if it's obscured. If the pigeon is skittish, I don't risk head shots. Ultimately, I need them dead/gone, but a center/high chest shot is effective too. Side/back shots~you have the structure of the wing/spine to get around so I don't take those with the Diana. A perfectly-placed shot=the pigeons don't even open their wings...they tip over and fall.
If budget allowed...or if I really loved shooting pigeons...I'd move up to a Diana 34 in 22 and just take any centre-mass shot and know the pellet is passing clear through. I'd still probably use wadcutters, they just bust things up.
The semi is a bad idea for a pile of reasons, not the least of which is how poorly CO2 performs in cold weather. I'd never leave a gun I cared about in a cold barn (or any outdoor space where temperature/humidity swings can be extreme) either, but that's me.
So basically, my vote for simple-to-use/powerful/quiet/dependable would be a .22, springer. Personally, I'd get a decent one...and am old-school. (preferring German) I have no experience with modern, .22 springers or any of the gas-strut/Nitro Piston things. PCP is too expensive/fussy for my tastes...and maybe too loud for in the barn.
Last note for your friend~forget the "quick follow-up" thing. Even VERY dumb pigeons will figure out you're after them...and will learn that once the first bird drops, they should hide...or get out of town. The more often you go after them, the quicker they do one of these...once you start shooting. My experience is that it's not a "one-time" pigeon clearing session...it's repeated/frequent visits to clear a few each time.
Anyhow, just my experience.
