[Uri’el-2012]

It could’ve been a 9mm, .45, or 7.62.

I would say
M4 => .223
MP5 => 9x19mm, 10mm, .40 S&W

[RightOnline]

Found this site (http://www.usnews.com/news/washington-whispers/articles/2011/05/11/the-gun-that-killed-osama-bin-laden-revealed"), stating (supposedly definitively) what was used for the kill:

"HK416 rifle, an M-16 type weapon....firing 5.56 mm rounds"