My guess would be thin, cheap plastic. I have reloaded 20 and 28 gauge AA compression formed years ago and would get 14 reloads especially with the 20 gauge. The only thing that ever split was the crimp after a while.
Now I just use Remington Gun Clubs in 12 gauge and they go longer but I have not had a crimp split on these. I prefer them over the Nitro Gold and STS. As to the Winchester AAHS not nearly as good as the old ones. For 12 gauge I will stick with the Remington's. I have not seen where cold affects the hulls.
