Objectfinder cannot find a blueprint

Here is an example on how to get the CharacterBlueprint assigned to the GameMode.
Thats from the FirstPersonShooter Tutorial.

static ConstructorHelpers::FObjectFinder<UBlueprint> PlayerPawnObject(TEXT("Blueprint'/Game/Blueprints/Character_And_HUD/BP_SurvivalCharacter.BP_SurvivalCharacter'"));
	if (PlayerPawnObject.Object != NULL)
	{
		DefaultPawnClass = (UClass*)PlayerPawnObject.Object->GeneratedClass;
	}

I guess you need to change a bit of your code.
The ObjectFinder is Searching for UBlueprint and not for the ABaseProjectile.
And the rest of the line should be what the reference says.

(TEXT(“Blueprint’/Game/Projectile/BaseProjectile/Blueprints/BaseProjectileBP.BaseProjectileBP’”));

Note that the cast in the if will differ from yours, because the DefaultPawnClass is a UClass and your class is ABaseProjectile.

(: Hope that helps you.