I've broken one. Didn't snap it off, but bent the fuq out of it, right at the receiver. Colt M4A1, too, issued gun. The only reason it didn't snap right off the gun was because the buffer stopped it from bending more - it bent as far as the buffer would allow sideways movement, and stopped there. I wouldn't call it "normal use" because I landed on it doing a PLF at the end of a 60-ft Fast Rope. But still - they're not impervious to getting fucked up. Everything has to be aligned, just right, and you can break anything.
A titanium castle nut and endplate wouldn't have stopped that from happening, though. That was HAPPENING, just in how it worked out.