That depends entirely on the gun. I'd stick with the lighter guns if you're playing though so you can move around easier and more quickly.
However if you want a more realistic feel, you can use a metal gun. They aren't much heavier at all. Most metal guns weigh roughly about or less than an actual firearm. For example an H&K USP .45 Cal. weighs in at 1.96 lbs. A full metal USP weighs in at 1.86 lbs. The point made by the previous answer also makes sense, but the weight isn't all that much. Plastic ones aren't as well constructed and will fall apart in a year with use. But they are lightweight and reasonably cheap.
An Airsoft gun
A pump airsoft gun is a gun that has to be pumped every time before shooting. Ex. Airsoft shotgun
No
If you bought a full metal one like me, it would be around 6-7 pounds
No
It is an airsoft gun based of the PPSH-41 gun
5-10 pounds
500+
1/4th of the flash hider
Check airsoftmegastore.com
Depending on the company $90 to $175.
You never need a license to have an Airsoft gun.