I was using Abbey Predator Ultra in 1 degree temps the other day, this is similar to Nuprol 2. I was only out for a few minutes so not enough time to cool the mags down, fuck being outside in that weather. It performed well, On full auto ROF obviously slowed down towards the end of the Mag.
I am new to the GBB game but I think Nuprol 2 is a relatively medium power Gas for 10+ degrees
If for some mad reason I was going to skirmish in weather colder than that, I would use Abbey Vertex or Nuprol 3,4