The PO-9 gas blowback pistol is a fantastic gun. If you are looking for a first airsoft pistol. i do recommend buying a gas-blow back (GBB) pistol. Get yourself a spring pistol and have a few games with that gun and then that will help you decide easier on a pistol and how much you are willing to spend on a sidearm.
If you have already done what i have suggested or are just really impatient like myself then i would suggest buying the PO-9. The PO9 is about $100 - $200. I don't know is that is a lot for a sidearm that you are only going to pull out once in a blue moon. Yes it really is worth spending the extra bit for a new gun.
Buying a GBB gun, second hand, is a huge NO. The reason for this is because a gas gun has moving parts and seals, and after a lot of use the parts wear and you will have to replace seals and go through a lot of trouble, so just spend the extra and buy it new.
Now onto the gun.
The PO-9 has a full polymer body with a metal slide and metal internals and also comes with a metal magazine. The polymer used on this gun is very high grade and is very strong and also durable. The PO-9 is also a very modifiable gun. There are a lot of after market parts, so the customization are endless. The gun shots around 300 - 350 FPS so its perfect for CQB battles.
Their are a lot of other pistol options but in my opinion the PO-9 by ASG is one of the best pistols and i currently run it as my sidearm and i couldn't be any happier.
link below to buy PO-9
