Pyramid Air is a source for many large caliber Airguns. They are most all made in China or Korea now.(they can't have real guns) I haven't seen any European guns in larger calibers but I know they exist, because the reason the Europeans (Specifically Germans, and Spaniards) are so good a making airguns is because they weren't allowed to build firearms after WW2, so they built airguns instead. If you can build a small caliber one right, you can also build a large caliber one.
I have a Weihrach HW35 in .177, and a Weihrach, HW77 in .22 both are very accurate but not suitable for anything above the size of a crow or squirrel. Not even close to a .22LR. Both run about 625FPS and are accurate to 40-50 yards, but best inside of 20 yds. I shoot my .177cal one in my machine shop just about everyday at 10yds/30ft. It has a peep sight and is good practice for big bore rifle shooting with iron sights. The .22 has a 4X Burris airgun scope on it.
I have seen several Precharged Pnuematic .45 and .50 cal guns that were shooting 175 gr roundballs at around 500-600 FPS. That's pretty powerful for an airgun. Certainly enough to kill a man. I've also seen a .50 cal pistol that was shooting the same 175gr round balls at 300fps and would completely flatten the ball into a quarter sized disc when shot into a steel plate. It would certainly get someones attention if hit by it.
Lewis and Clark carried a .62cal Airgun on their trip to Oregon a couple hundred years ago. It was purported to be very powerful, and fully capable of killing a man at 100 yards. They killed deer and elk with it also. It took 3,000 strokes with the pump to charge it fully and then it was good for several(8-10) full power shots. We have SCUBA tanks now.
Napolean gave the death sentence to anyone caught with an airgun on or near the battlefield. Silent Snipers, and no position revealing smoke. All the good guns were Swiss made at that time
I haven't seen one of the "oriental" guns up close, but I think they have a ways to go quality wise before I'd buy one. Especially for $500-700. The pics of them don't look "that" good. I could be wrong here,,,,But it's too bad they aren't made by the Japanese? Miroku makes nice guns, I have had several.
I would do a search for custom airgun makers in this counry. I know there are several guys who make them and the .45 and .50 cal rifles are "real guns" , not toys. .38/ 9mm is another popular caliber that would have some bite to it. Below that it's just a pellet gun.
I once shot a guy who was bent over rifling thru my car in the middle of the nite with my very powerful sling shot, using a cast .44 cal 240gr SWC. About 15 feet was the distance, and I shot him directly in the ass . He banged his head real hard on the roof of the car and turned around with the biggest WTF look on his face I have ever seen, I asked him if he'd like another, and I have never seen anyone exit stage left as fast as that bum did! I can only imagine the size of the welt on his ass the next day. A big pellet gun would have probably given him something to really remember me by.
Sometime life dishes out delicious retribution. Savor it when you can, as it is far more satisfying than shooting rats.
I have seen and fired a 9mm air rifle. I am pretty sure it was Chinese made and the brand was "Career" I think the pellet was only one third the weight of a 9mm pistol bullet, but it was pretty accurate. One thing that surprised me is that it had a pretty significant kick, not as much as a 30-06, but a lot more than a .22 rimfire. There are some air rifle web sites, that can get as expensive as a good target centerfire.