Have you ever fancied a gas rifle that would be fine to use in the cold? Have a sound report that wasn't easily mistaken for a sewing machine? Be compatible with your current AEG magazines? Adjustable recoil, fps and rate of fire? Check out www.airsoftgasblowbacks.com
Currently there are AKs, M249s and MP5 and even drop in kits available so perhaps you convert that Top, Classic Army, or A&K M249 to make into a real bucking beast and really give the other team something to be scared by!
"All of our GBBs run on Nitrogen, by far the best gas available. Not only does it have a crisp sound but runs clean. It is regularly available and is usually free.
The good thing is Nitrogen can be used in freezing weather. You can literally pull the trigger for minutes on end with no cool down effect. Note..You can also use co2 with this system.
We use an external power source so there is no need to worry about gas spitting, cool down,or refilling in the middle of a game.
The rig has a female stabilizer connected to your Nitrogen or co2. The line is hidden in your sling or camel line to be concealed. You can also blend it into the webbing making it look like a comm line. We have done it this way because you dont see or notice it and acts just like a sling. 
The air rig is also available here for first time buyers. You can order the training system with or with out one. "
More at http://www.airsoftgasblowbacks.com/journal.html
- James The Camel