I was out shooting my 17s today and doing the 50 round 5 shot barrel break in. I had the rifle and ammo (PMC Nato) out in the cold 18 degree weather for about an hour as I was shooting the 5 shot groups. I noticed that on the last 15 rounds the rifle would not lock to the rear after the last round from the magazine was fired, and also the rifle would not always strip a new round from the magazine, it also had trouble fully going into battery if it did manage to get a new round from the magazine. I figured the rifle was short stroking but am wondering what would cause this? Is it because the cartridges performance would have been diminished from being in the cold for an hour? During the first 35 rounds there was no problem with the rifle functioning. I have the BCG lubed with the EWL grease. Thanks for any replies.