They may be back... keep your fingers crossed.
There was a guy at the Dianawerk forum that took a couple of Beavers with head shots 30yds out using his Diana 350 .177. It was actually a 16yr old kid. A healthy 350 .177 has about 19fpe at the muzzle with heavies.I know Beavers are large animals. Generally bigger than a raccoon and can reach 35lbs or more. However, a precise shot to the brain will do the job from a reasonable distance, if you hit it with at least 11-12fpe on impact - at the right spot. Coyotes have been taken with less. With that said I would personally get up close to within 20yds with a Disco .22 or 350, or other Ultra-Magnum Springer, to ensure proper shot placement. That is, if I had no other more powerful gun at my disposal. I would definitely use the most powerful one I had and preferably one with >20fpe.
Trevor, I am not sure what you are disagreeing with. What I said about someone taking beaver with a Diana 350 is true as far as I can tell. The guy posted pictures with his 350 and 1 pic I remember he was holding up the beaver in 1 hand (struggling it looked like), and the 350 in the other. He is from Canada IIRC. He said they were 30+lb beavers. If it were a 40-50lb beaver I would probably not use a 350, even if that's all I had. I agree with most here. I would use at minimum a 30+fpe PCP, and if I ran into one today that needed to be dispatched, you can be sure I would use my Condor .25 with a 31gr Kodiak dialed up at full-power at 80fpe. In any case, it takes 3.5-4fpe on impact to crack a human skull, and to kill a squirrel with a head shot. Many have taken squirrel with their 5fpe Crosman 760 pumpers including myself at 10-15yds. I thought the 10-15lb Groundhogs I have around here had very thick skulls, but I had complete passthrough's using my 14fpe Diana 36 .177 with 7.9-8.3gr pellets at 20-25yds. A guy in another forum (I will just say his name is Dave) shot himself in the back of his ankle by accident with his detuned 7.5fpe Diana 34 .177. The pellet went in >0.5" through tough muscle and tissue and came very close to the bone. He had to have it surgically removed. He posted his X-Ray on that forum showing the pellet in his leg. Please do not underestimate the power of airguns. Raccoons do not need to be shot 6-7 times in the head to be killed with a 15fpe airgun. Your shots were probably not accurate and missed the brain, or maybe you need to get a Chrony and see exactly what your Whisper is doing. Shadow/Ed and others here have taken many raccoons with a single shot to the head from 20+yds with 15fpe guns, some of them Whispers .177. I have taken raccoon and possum with my 350 .22, 1-shot 1-kill with head shots. This 350 .22 is "detuned" to shoot at 21fpe (instead of 22-24fpe) with JSB 15.9gr. Many hunters here have taken raccoon with 14-15fpe .177 & .22 Springers. Skull bones are not as thick or tough as wood or other backstops that we often use. Once the pellet hits the right spot from a reasonable distance, the pellet will go through the raccoon's skull and damage the brain if the shot is accurate. I am not so sure about shots in a raccoon's ear that far back. I have never shot a raccoon in the ear and that is not my preferred shot. I always go for the frontal shot "between the eyes", and on 2 occasions I took raccoons with a broadside shot, I always aimed 3/4" in front of the ear and at the upper half of the skull very close to the top. I really believe that beavers have a "hard skull" if they reach 45+lbs. So do Coyotes, but they have been taken with 14-15fpe Diana 34's .177, and other Springers with a single shot "between" the eyes at 15yds and beyond. Those guns would not be my choice of gun for a Coyote either, but I am just stating facts. If I were going after Beaver (or Coyote), I would use nothing short of my .22 or .25 Condors.