some people complain of gearbox fractures due to cold weather. i've played with my systema gear box for a few winters with no trouble, but if you find you're cracking gearboxes, or just don't want to take the risk, Garber My Coal makes a toe warmer with an adhesive backing. stick this to the side the a metal body of your AEG. the closer you get it to the front of the cylinder, the better as this is where gearboxes tend to crack. to further increase the surface area of the warmer in contact with your gun's body, secure it to the body with electrical tape. the tape will also provide some insulation, directing heat to the body (path of least resistance).
batteries do not create chemical reactions (that create energy) as quickly, or as efficiently, in the cold. stick one of these in your battery compartment, but make sure your battery has an operating temperature somewhat beyond the warmer's max temperature of 107 (avg 100). this max temp measurement was probably not conducted in a small enclosed space. don't stick it directly to the battery, rather someplace where direct contact will be avoided. the ambient temperature create by these warmers is much less than contact temp.
these warmers last 6 hours, and cost a buck (i believe). two toe warmers per package.