I think it works, you just do not see the effects cause there are a lot of movement and slugs.
I have changed in your cap the accuracy calculating to "(Sprite.Value('accuracy') + Chair.value('modifier') )* Chair.Value('speed')"
and attached the action to the spawn moment of the Slug.
Now, you can test like this:
Do not move the cursor: the bullets are fired in one straight line
Move the cursor next to the soldier and then fire it while moving only vertical. You will see the slugs disperse more.
You will only have to set the weighting for the speed modifier.