What's That Bump Behind The Ejection Port On An AR-15/M16?

I'm not so much of an AR lover, but that's changing the more my knowledge expands from "they sucked in vietnam (for various reasons)" to "They're a mature technology, and highly-refined at that." but that's not what this post is about. I've just had a "eureka!" moment.

There's a bump behind the ejection port on an AR, and I had heard it was a shell deflector, but I didn't really get why that ugly lump was hanging out on what seems an otherwise 100% function type of machine. Then I watched the slow-motion video of the ejection of spent brass from an AR, and it became crystal clear:

The empty brass slams into that bump
every.
single.
time.
and goes flying off to the right.


Image lifted from GunAccessories.com.