CO2 Non BlowBack pistols are the easiest gas airsoft pistols to work with. These pistols are 1911 styled and have a fixed slide. all you need to do is load the magazine and pull the trigger to fire. 
These pistols do shoot a bit hot, around 450FPS with .20g BBs, so they are only recommended for backyard plinking.
We recommend heavy BBs for better accuracy.
Note: CO2 pistols live relatively short lifespans. CO2 is a very high pressure gas, is unfiltered, and is also dry. Particles from CO2 capsules can deform the hard rubber and plastic seals used in CO2 valves. You may have to replace the magazine periodically to keep this pistol in working condition and every time you put a new CO2 capsule in the magazine, make sure to put a drop of 100% silicon lube on the puncture valve.