I was looking at this new keystone and figured it could work well for several melee supports with some good CC, Braum being one. My question is, if the 4th stack of Braum's Concussive Blows is procced by an ally, does Braum still get the shield? It would make a fairly big difference in whether or not I take the mastery on Braum as is only other hard CC is his ultimate.
Yes, since the source of the stun and proc damage is always Braum. I'm not really convinced CoC is optimal on him, but it's worth trying to see how it feels.
It is worth a try, especially since 
pretty much emulates/improves on Bond of Stone (minus the raw damage reduction on oneself, which is nothing to scoff at!).
It is, which is part of why I think it's sort of bad on him. Too delayed, and it can be easily procced in lane at a bad time putting it on cooldown when you actually need it. I'll be sticking with Grasp because I hate Bond of Stone :P
I love that mastery on braum. It's delayed just enough to make you look like you're going to die, but then get saved by the shield. Enemies spend too much time focusing you and not enough on your teammates because they think you're low.
Also, some of the counterplay to coc is to bait it out by faking an engage, but Braum's can't really be baited. His q is only soft cc until he actually engages.
The mastery is better than grasp provided that you have plenty of health regen. I take health regen quints and build about 400% bonus every game, so I don't really need the sustain of grasp, and you get more damage while the shield is up because you can go more aggro.
To answer your question, yes. Even though the fourth auto can be applied by any of your teammates, they deal the magic damage and Braum actually applies the stun.
Yes
