I’m not sure how the projectiles are causing damage without looking at the BP for the projectile in that tutorial, but another way you could do it is like below with Tags.
Using the First Person Template as an example, tagging the Projectile with something and doing an Overlap Check for the actor with that Tag, you can reduce “Health” that way too.
There’s a bunch of different ways you could handle it. As for your example, you’d have to look at the collision properties of the barrel you are shooting at and potentially the projectile as well and see what they are set to.
Hope this helps,
-W