There are lots of opinions on what is the best pellet. I think some of the confusion is created by pellet manufacturers marketing expanding pellets as hunting pellets. I can say from personal experience that expanding pellets are not at all necessary to cleanly kill small animals. Like squirrels. A .177 caliber hole through a squirrel damages the same percentage area of the animal as a .75 inch hole in a deer. It is by calculation and practical experience a big enough hole to cleanly take the animal.
But the hole has to be in the right place. That is why accuracy is so important. My guns seem to either like H&N or JSB pellets, not both (there is one exception). I like my velocity to be 800-900 fps when possible so I try pellets that will give me that sort of velocity in the subject gun. I can retune to go up or down but I start with them tuned as they came. I tweak the hammer spring setting on my regulated guns to get all the air out of the regulated chamber but I don't change the regulator until I see a reason to. For a light pellet in a 22, I will test H&N FTT and a similar weight JSB offering. For a heavy, I test a JSB 18 and a H&N Baracuda. Most of my guns prefer H&Ns but my P35-22 shoots JSBs almost as well. JSB also makes FX and many other brands of pellets and they are a little different so it can pay to test them soon. But I think the first thing to know is if they like the slightly harder H&Ns or the slightly softer JSBs. My Avenger really does not like the H&Ns but shoots nearly any JSB pretty well. FX the best of the ones I've tested.
I see the role of an expanding pellet to minimize pass throughs or energy for a pass through. A 22 caliber PCP under 20 fpe will not often exit a squirrel but my 32 fpe 25 caliber PCP will almost always exit. If I did not want it to, I could try and find an expanding pellet that it shoots well and use that instead. It might also kill a little quicker but if a 177 size hole is adequate, a 25 is even more so. So I do not see the expansion as very important to the lethality, only the tendency to "over penetrate". My limited 177 experience indicates that over penetration is not a big risk because the pellets are normally under 12 grains and even at 900 fps they are under 20 fpe. I've shot through a couple squirrels with mine but the pellet also stopped in a couple others (all died quickly). But higher powered 22s and 25s can certainly shoot through BTRs with a simple domed pellet. They will die quickly but the pellet may still have enough energy to damage something when it exits. I concentrating on my 177 right now but I may use some metal mags in my P35-22 to take some squirrels to see how it does. It penetrates further in wet paper than my P35-25 so I am confident it will usually go through a squirrel if I use the Baracuda Match it likes. It also shoots metal mags pretty well, however. They should penetrate less but make a wider hole. Might be a "good hunting pellet" for this gun. But a Baracuda Match would still remain a viable choice for hunting. As long as I put the pellet into the vitals, I am confident either will take the squirrels cleanly but I still like taking a few to see it happen.