Loss of velocity is a sure sign of a compression issue. When it only happens in the cold, that's usually due to the piston o-ring shrinking a tad in the lower temps. Double feeding in the cold is typically the same rubber-parts-underperform-in-the-cold issue, but the culprit being the hopup bucking. The worst is when you get both problems at once as I saw happen with someone's prized Masada. A dream in warm weather, but on cold mornings in November...
Piston O-Ring: Take apart your gearbox to get to the piston/cylinder assembly. If you have a spare cylinder laying around, great. Take off the piston o-ring and soak it really good in silicon oil (no petroleum based oils). Stretch it over your cylinder and very lightly heat the metal cylinder over a candle (don't cook the o-ring directly) until the cylinder gets warm to the touch. Set aside to cool. This will not only help to stretch out the o-ring ever so slightly, it'll help it retain some of this new shape so you don't have to repeat the process every time you go out in the cold. If you ruin your o-ring (don't be ashamed), take a couple bucks to the hardware store and get yourself a box of #14s.
Hop-Up Bucking: This is trickier. Replacement is the only solution I know of. I've been trying various buckings and haven't had any double feeds in the cold. Gaurder's black and clear both worked, Systema was fine but took a LOT of breaking in. My blue Madbull that came with their Ultimate Hopup Chamber works fine. I'm in the process of breaking in a purple Prommy, but haven't done a cold test on it yet. I've only seen a hop-up bucking act up in the cold twice, including yours. So my guess is that it's pretty uncommon and fairly exclusive to buckings that are on their last legs.
Ken does a lot of tinkering, too. Hopefully he'll chime in with some input as well.
There are a lot of things that can contribute to the compression/lower FPS issues in the extreme cold conditions. As Joe mentioned it does have a lot to due with the piston head o-ring. The reason some guns work alright in these conditions and some don't is due to the quality of material the o-ring is made of as well as what type of lubricants were used in the gearbox. In the extreme cold the greases inside of the metal casing can get very thick and cause much more friction on all of the moving parts inside the gearbox as well as cause more strain on your motor and battery an so on. There are a couple of things to do when it comes to playing in these cold conditions. One is to make sure your gearbox is properly greased with quality grease and also make sure it is shimmed properly. The other thing you can do is to install a lighter spring, try installing a M100 which will reduce the strain on your gearbox as well as your motor and battery.
The bucking also has a lot to due with both of the conditions mentioned, loss of compression as well as the double feeding. Most of your guns come new with a "hard type" bucking usually 60% which has less grip on the bb in normal weather so in the extreme cold it becomes even harder so even less grip!!! As Joe said the Prometheus purple, soft type bucking, 40% is a much softer bucking which will keep a better air seal as well as a much better grip on the bb which should correct the double feeding issue as well as help with the air seal. Systema also makes a real good soft type bucking, this is the one I have been using in 2 of our M4's and has performed well in all weather conditions. I have just installed one of the Prommy purple buckings in my primary M4 and have not had any issues with it in the cold weather.
With the 2 types of buckings the harder bucking will last much longer but will not perform near as well as the softer bucking. The softer bucking will perform much better but will not last as long until it will need replaced! We also have one of the blue Madbull buckings, which is a soft type, in Josh's M4 and it work well in all weather conditiones. My choice going forward when I have to replace a bucking is to go with the Prommy purple and I do not use the nub that comes with the Prommy, I use one of the SCS concave nubs which seems to work really well with all of the soft type buckings that we are currently using.
I my previous post I referred to the 60% or degree, as a hard type bucking, the 60 is the normal bucking and the 80% or degree is actually the hard type. I have still found the best bucking for AEG's under 400 FPS is the soft type, which is the 40%.
