Unless you've shot your airgun through a chronograph, you don't really know what its doing. The advertised velocities of break-barrel guns is almost always off. Its probably doing something like 300-400fps or less if it isn't shooting deep into the coon. Basically like a Red-Ryder. Break-action airguns often are cheaply made and the seals go bad on them easy. My brother brought me one last week that we choreographed at under 400fps, but you couldn't tell it from shot noise or watching the impact.
A standard weight .177 pellet impacting at 850fps should be able to penetrate a coon's skull pretty easy presuming it doesn't hit an angle and deflect. At an impact velocity of 850fps, a .177 pellet will provide about 7 inches of penetration into ballistics gel. The formula for calculating penetration into ballistics gel is projectile weight x 100 / 7000 /.caliber/.caliber x 2.06 x impact velocity (expressed as 1.0 for 1000fps). The formula is conservative if anything. If you aren't getting penetration equal or more than what the formula would predict, your velocity isn't as fast as you think it is.
If you browse around on my channel, you'll see video documentation of scores of clean kills using PCP air-rifles on all manner of larger game, including raccoons, beaver, fox, whitetail deer, wild turkey, and wild hog with calibers ranging from .177 to .45.
Here's a wound channel into hard, thick, wet, clay at 50 yards with my .308 airgun. The wound channel was 10 inches deep and my fist could fit inside the largest part of the cavity. That will kill the heck out of a deer or a coyote, much less any varmint that would be prowling around the coop. Its gross overkill on a chicken, but I have used this gun to cull chickens with the power turned down to shoot light pellets. I don't find the .177 or the .22s any less effective so I see no reason not to use them for simply culling.
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