if it coming down the barrel, then its most likely your o rings on the valve body,

valve stem springs dont usally go like that unless its been fiddled with

there is also a chance if its a regged gun it my be venting air through the small breather on the reg, just put a little spit over it and see if its leaking.

O ring kits available for a service, very simple if your careful, and the reg is quite simple but you will need to check the reg setting after to make sure its still set correctly.

don't panic its easly sorted

the bottle valve is always open when on the gun so a leaky bottle will not be the problem.