Great bag if you have an M4 or similar. Use this one for my G&G Cm16 carbine. Fits in nicely with the stock unextended and can get a large bottle of ammo in the end with it. The pocket holds 4 M4 mags in the large bit and my two batteries (plenty of spare space in this bit) in the small front pocket.
This won't hold anything bulkier (like an F2000) as it does narrow quite quickly. I would recommend getting something wider if you have any attachments that you don't want to remove each time you put it in the bag.