Ian and Karl at InRangeTV have been doing mud tests on several rifles.
So far most have failed miserably (including the AK ... yes you read that right the AK ... but it gets worse for you commie gun lovers*, the AR passed with flying colors).
Links to some of the other vids (no sense in embedding all of them)
M1 Garand
Full review of KelTec RDB (new 556 bullpup ... includes mud test but is about an hour long ... mud test runs from 30:27 to 45:28)
M1A vs MAS 49/56 vs AR (slightly different methodology than the others ... oh and the TL;DR version is that the MAS and AR do well ... M1A not so much)
Now we sit and wait for the AK fanboys to come unglued and find all sorts of fault with the test.
* That's lovers of a "commie gun" not commies that love guns.