1. Re SHTF: What JohnKSA said.
2. Joshua, thanks for the info! Extremely interesting
3. A Barnes Delta Ranger in .32 or Quackenbush Exile in .308 (PCP guns) would be my choice for a "large" bore defensive arm *IF* the gov't banned guns altogether, but would not be my choice for SHTF, if .22lr's etc. are available because of what Johnk said (hassles/availabitity of compressors, pumps, etc.)
4. The Air Force (name brand) "Condor" rifle meets or exceeds the RWS/Sam Yang Career 707, in power, and is a heck of a lot prettier, and lighter, and American made, and more modular (capable of simple replacement of the buttstock reservoir rather than a recharge), and has multiple rails for accessories, and a Lothar Walther barrel. Oh yeah, and it costs about 1/3rd the price of a Beeman Super 12. It's is the pig's feces in *smallbore* PCPs (.22), IMO. (at least on paper; I don't have one...yet)
http://www.pyramidair.com/cgi-bin/show.pl?cmd_items=1&Manufacturer=Air Force&Type=1
5. A spring piston gun in .177 or .22 would be preferable to a PCP airgun in any caliber, for the game-getting/survival aspects of SHTF, because it's human powered (simple cocking) instead of compressed air use, but this would still not be a good choice for defense vs. humans. And a .22lr would be better still than either type of airgun for any aspect of SHTF than a comparable airgun, particularly game-getting. Except, I suppose you could come to the conclusion that IF you're equipped with generator and compressor, then a PCP such as a Barnes etc. in .32 or 45 or whatehaveyou would be preferable to a .22lr for defense, to be sure. But if you have access to a .22lr, then you should also have access to big bore centerfire rifles (UNLESS the gov't, tween now and then, outlaws all guns EXCEPT for .22s.....hmmmmm). In any event, it's probably easier to stockpile/store/hide ammo in all calibers than it would be to maintain a generator and compressor, feed same with gasoline, oil, parts, etc., over time. I suppose eventually you'd run out of ammo, but eventually your compressor would break as well. Good reason to stockpile ammo..
6. Springers and PCPs are not quiet, as mentioned. And PCPs are generally lounder than springers. A .22 super colibri is much quieter than a full-powered springer or PCP. A .22 CB long is approximately the same "loudness" as a springer or PCP. Good thing about PCPs though, is that they can be dialed in to the power level you want, with corresponding report. The ultimate in accurate stealth if you don't have a firearm silencer may be that Air Force Talon SS (suppressed PCP airgun), which has a nice dial to adjust the power level (as do all Air Force models).