I have included a link to a youtube video showing the problem in better detail than I can describe, but basically bullets shot from the Mosin pass through infected and impact the ground behind them. No scopes, I was on an elevated position and, as shown in the video, I took two center mass shots with no effect. They didn't even react to the sound, until much later.

Here is the link: https://www.youtube.com/watch?v=SaIEQQ-nq40