And people say BB's are inaccurate. Tell that to the bird I pegged in the head from probably 30 feet away with a Red Ryder (that mockingbird on the chimney wouldn't shut the hell up and was squawking there every single morning). No, there's no
technical reason why a BB gun can't be very accurate, but nobody makes high-end BB guns because it's not politically correct (or even legal) for children to use a BB gun unsupervised anymore. All of them nowadays are just weak spring-powered guns like my Red Ryder there. Accurate, but gutless.
Some people like the Blue Streak / Silver Streak. I'm not a fan. My father has one, and I've used it plenty of times. It's heavy, extremely hard to pump, iron sights suck, and the stock has absolutely the worst ergonomics I've ever experienced in a rifle-style weapon. No thanks.
I think a precharged pump is going to be your best bet. An inventor from the paintball world came up with something a while back you'll find interesting: it's a booster pump for a shop compressor that, in a few hours, can charge a 68 cubic inch or so 3000 or 4500 PSI air tank. It's relatively quiet, and costs $550. That could certainly solve your problem of relying on a SCUBA tank. Check it out here:
http://www.shoeboxcompressor.com/
Here's the fill times listed:
•3 minutes to fill a one cubic inch air tank to 4500 psi or 300 bar
•68 cubic inch paintball tank 4500 psi = 3.5 hours
•80 cubic foot scuba tank to 3000 psi or 200 bar = 24 hours
•Marauder air rifle 22 cal. 0-2800 psi = 18 minutes
•44 cubic foot SCBA tank from 3000-4500 psi = 3.5 hours
Note: I know enough about the person behind this product to know that he won't stamp his name on something that's junk or that won't live up to hype.