How to correctly blend Shooting animation with Running animation

I have a shooting animation where the (humanoid) character just raises their arm 90 degrees and repeatedly experiences some recoil (to mimic shooting a gun). Then I have a running animation where the arms are moving back and forth, just like in a regular running motion.

Now when I add a new animation layer and apply an arm mask for my shooting animation, and if I run and shoot at the same time, the arms do as expected, but since the running animation rotates the torso with each step, the arms look like they are shooting everywhere.

How can I make the arms point to the correct spot while keeping the running animation?

I've tried locking the position and torso for my mask as well, but that just results in a weird running animation where the torso doesn't move.

Thanks!