I had exactly the same issue as T-Hell. The bb's came "FRESH" out of the bag (which had been stored in my basement) and into the mags, 1.5 hours later they were jamming every mag it had, TM M14 AEG and S&W 40F pistol.
Every bb that came out of the mags, later that day, had a dimple in two spots (top and bottom) and some of the bbs from my pistol came out is worse shape than that.
In my pistol mags the bbs had tried to pass each other. Instead of the staggered arrangement they ended up side by side. Had to use a dental pick to remove them from the mag.
I've always used bastards and never had this problem with any of these mags, heat or no heat!
Nothing says loving, like a head shot!