Some of the big brakes have a lot of force. if you go to 1:11 in my 50BMG video you can see how much blast is being redirected to the sides. Enclose the area around it as in the video Stryker posted, and it's going to be amplified:
I hate it when people don't watch there brass or take into account the rifle they are shooting. I remember this jerk started shooting a 300win mag next to two kids and there dad. They were probably 10yrs old and shooting a bolt action .22lr. They were getting freaked out and scared. I was to the left of him and figured he didn't know the left side of the outdoor range was kinda for high cal and the right side low cal. So I told him. His response was "it's a free country" I thought for a moment and pulled my semi-auto rifle out and emptied two mags of .223. Letting it rain brass on him. He of course complained and I responded "it's a free country man" He packed up and left in a huff. I thought I handled it poorly, but after the range was called cold a bunch of people came over and thanked me.