Conducted many experiments with mags, springs, etc, the ammo is the one constant that keeps nose diving. When I use factory ammo that does not have the hourglass it runs like a champ, when I use my buddies ammo that does not have the same amount of hourglass it runs like a champ, so it is my ammo. The only difference is the shape of the brass since we have the same bullet and OAL.
As I mention ones in the OP he uses a Hornady sizing die, his brass has the same taper as factory brass.
I was thinking about the shape of the brass. If you take your hands and hold them up so you can see the back of your hands, then put your thumbs together at the knuckles, there is likely a tiny little gap you can see daylight through, now slide one hand up long the knuckle. See how they interlock and then force must overcome that locking. I think this is exactly what is happening in the mag.