I'll go ahead and give you the entire explanation. Heavy Ammo Finder is a mod you can put on helmets that grants a small chance of finding additional heavy ammo bricks from enemies that you defeat. These ammo bricks do not behave as regular ones. They typically grant less ammo than regular ammo bricks and they have their own separate drop chance when you kill enemies. There are two ways to tell that Heavy Ammo Finder is working aside from picking up the ammo bricks. When you see that "Ammo Finder" buff text appear on the left side of the screen during gameplay, that means that an enemy you just defeated dropped one of these extra ammo bricks. Another way to tell is that the ammo bricks that drop due to Heavy Ammo Finder also glow a bit more than regular heavy ammo bricks. All Ammo Finder perks will only work if you have the appropriate weapons/weapon types equipped. Heavy Ammo Finder will work all of the time because you will always have a heavy weapon equipped. Special Ammo Finder and Primary Ammo Finder will only work if you have those respective weapon types equipped. And the specific mods like Sniper Rifle Ammo Finder and such will only work if you have those specific kinds of weapons equipped. In some cases, if you have something like a Sniper Rifle Ammo Finder mod while you have both heavy and special sniper rifles equipped at the same time, you'll have a chance to get extra special and heavy ammo bricks to drop from enemies. They always drop as separate types of ammo bricks though, so you will never pick up a single ammo brick and get both special and heavy ammo at the same time unless other conditions are met. And there is no guarantee that both types of ammo bricks will drop at the same time either. One last thing to note is that when the Famine modifier (decreases the chances of finding ammo bricks) is active in an activity, these Ammo Finder mods are pretty much unaffected by it, so they can be really helpful in some situations. But these mods don't work in the Crucible.