Hi Chuck,
Gas pistons or "gas rams" have been around for awhile. Gas rams only tend to come in high-end airguns, but that doesn't mean you can't by a springer of equal quality.
Spring guns inherently have a few problems:
-cannot be left cocked for any long duration of time, or you MAY fatigue the spring.
-spring guns are scope wreckers...you need a special spring airgun rated scope for springers. They will destroy a $1000 high-end big game scope rather nicely.
-recoil. I know you're all laughing at this one, but spring guns have multi-directional recoil from the release of the spring...this makes them the most difficult airguns to shoot accurately.
I would seriously consider a pre-charged pneumatic. They are near zero recoil, making them exceptionally accurate. Plus they are significantly more powerful then any other type of airgun and get several shots per fill. Most all of them come with excellent adjustable triggers. These guns will usually make daisys and gamos seem like toys.
Consider this one:
Benjamin Discovery Air Rifle. Air rifles - PyramydAir.com
When it comes to pre-charged airguns, I would recommend .22 or .25 caliber over .177. Just my opinion, but they generally hit harder and are less affected by wind. The benjamin discovery can reliably hit a half dollar at 100 yards. And if I'm not mistaken its made in the USA. You can fill it up using a hand pump, or a scuba tank, or run it off the smaller CO2 cartridges.
Nothing even comes close to the quality and power of this airgun for the $.
If you want multishot capability, you would need to step up to the Benjamin Marauder:
Benjamin Marauder Air Rifle. Air rifles - PyramydAir.com
A bit more pricey, but it's 8 shot magazine in .25 caliber, excellent trigger and it arguably the best sub $500 airgun available.
So I was reading up on air rifles and see that in the break barrel types there seem to be at least two different mechanisms for the piston. One reads as a spring piston while the other is a gas piston. Is that one of the break points separating the higher end guns from the lower end?
Chuck