Cold weather may harden any lubricant in the bolts internals slowing down the firing pin. Dry fire it in the house and listen for the "lock time" it should be a very quick whack as the firing pin hits home. If you get a FTF in cold weather again dry fire it and see if it repeats or if the sound is a slower duller impact.The norinco ammo may have had harder primers. If that is the problem you could use a lighter lubricant that is formulated for that temperature range to solve the problem.
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