Of all the rifles you listed and had experience with but failed to meet your definition of accuracy, I’ve never shot any of them at all, except the Air Arms rifles. Were they springers or PCPs? I own a TX200 springer and an S410 PCP, both from Air Arms, and I can tell you that both are as accurate as one another. The only limiting factor is the shooter, in my opinion. I also own a Cricket (bullpup) that is also as accurate as the two Air Arms rifles. It’s just easier to shoot for me, though, because of its ergonomics and the way they suit my shooting style. I also have owned a first-generation Airforce Condor, which I sold a long time ago. That too was as accurate as all the ones I currently own. Though not included in your list, I owned a Mendoza RM22 spring rifle, which had what I’d estimate to be a 10-pound trigger-pull weight.
Now, if we are going to narrow it down to precision at long-range shooting, I will say that the Cricket will be the one that will come out on top if I were to put them up against each other, with me being the shooter. Why? It’s because the Cricket is regulated. Having a regulator really helps, especially at long ranges. But even with this, an unregulated PCP will still be just as accurate as a regulated one if you are able to keep within the consistent region of its power curve. Nine years ago, I took down a pigeon at past 100 yards with my Condor on the first shot. Having no laser rangefinder at the time, I had to estimate to the best of my ability the distance between the tip of the Condor’s muzzle to the roof the pigeon perched itself on. Fortunately, I had a printout of mildot aim points courtesy of Chairgun taped to my Condor according to the velocity and pellet I used. I also was shooting within the consistent curve of the rifle. When the shot broke it took about a good couple of seconds to confirm the impact of the Kodiak 21-grain pellet smacking the back of the bird. Yes, even at that distance I heard the “thwack” that confirmed the sound of the death blow back to me. I remember the pigeon flapping its wings only once and then it dropped down into the bush below it. You say that your accuracy definition is minute-of- squirrel head. Imagine if I had a laser rangefinder with me and ranged the pigeon’s exact distance at the time.
All the guns I’ve stated I owned and currently own are in .22 caliber. It happens to be my favorite caliber, and it probably will be the only caliber I’ll shoot with airguns. All have been accurate tools to me, and let me say that my standard of airgun performance involves repeated, pinpoint (same-pellet hole) precision. If it’s any indication, so far, the caliber doesn’t determine accuracy. I will say this, though, that the Mendoza springer I owned—which was basically manufactured by Crosman and used Crosman’s barrels—had a barrel that had an oversized internal diameter. Regardless of what pellet I inserted into it they would almost just fall through. Combine this negative aspect with the very heavy trigger-pull weight, you can see why I sold it. That was the only inaccurate rifle I owned, but I was able to immediately associate its inaccuracy to these glaring culprits.
The rifle recommendations made here, of which some include what I had and currently own, are all excellent ones. Obviously, because I have one, a Cricket will most definitely meet your requirements for accuracy. But I see you have set your aim (excuse the pun) on a Mutant, which I've read about is also a very capable bullpup.
