Why get co2 for backyard use?I would suggest staying away from co2 blowback do to the slides usaly end up cracking or splitting alos end up messing up the gun itself.I would go green gas NON blowback if your just messing around in your backyard.
Don't get c02 blowback get gas blowback better for competitive play and cheaper gas. I recommend m93r from KWA all KWA gas blow back pistols are great! M93R has great power easy to maintain and clean. But it's heavier than the others and uses a lot more than others. That's somes it up u can find 93r 50-120 $ used or sometimes cheaper.
Seriously, don't get C02. Most of them are of lower quality and will break a lot faster. When a seal goes on a C02 pistol it's cheaper to buy a new gun than find and buy parts.
My suggestion? Get one of the new WE pistols. They make a few Glock models, the Sig Sauer 226, M&P pistols, and a handful of others. I own the Glock 17, Glock 18c, Sig 226 and I'm probably getting the M&P.