If you're gonna be plinking in your garden, with an eye toward paper punching, a single stroke pneumatic like the HW40 could be for you,
Pros:
accurate - no recoil, rifled barrel
Low running costs - no co2 bulbs to fork out for.
Well engineered quality air pistol, it'll shoot as well as you can at paper targets too.
Cons:
You have to pump it manually for every shot, thus no multi round fun.
If you're looking to plink/paper punch and determined to go for co2, Umarex have a decent range of multishot pellet pistols with rifled barrels, the S & W revolver 6" being highly recommended.
Pros:
Full metal replica of the S & W revolver (though does have a 'dummy' cylinder)
accurate - no recoil, rifled barrel
multishot - 10 shots before reload, quick mag swaps, around 50 shots per c02 bulb
Cons:
You have to shell out for the co2
Co2 is dependent on temperature, the colder it gets, the lower the power, and it cools with every shot.
If you're looking to get a co2 replica with blowback function, and have plenty of empty cans to shoot, you could get a KWC Sig Sauer P226 X-Five
Pros:
Faithful full metal reproduction of the real P226
Blowback, locks and loads like the real steel
Multishot, 27 shots per mag, quick mag swaps, 60 odd shots per c02 bulb
Great Fun for plinking
Cons:
It'll cost you for the co2.
Only fires steel/copper BBs not pellets, and through a smoothbore barrel, which will affect accuracy
Blowback eats the co2, so lower power, again accuracy suffers
Different strokes for different folks