this started happening to me today on the 5 servers I played on - often I'd shoot the enemy 9 times with the rifle and they don't die. I know the shot hit since little red particles comes off them indicating a hit, but it doesn't kill. I don't recall the rifle doing so little damage that it takes 9 shots to kill someone.
And the shotgun too, people were surviving 4, 5, to 6 hits dead center, at close range (under 20 blocks).
I thought it might be lag but my ping was 38-40ms.
It's probably a bug??