How to assign a TSubclassOf from a Blueprint class in C++?

From my ProjectilePak.h:

UPROPERTY(EditDefaultsOnly, Category = Projectile) 
TSubclassOf<class AFPWeaponsTestProjectile> ProjectileClass; //Projectile class to spawn

I am trying to assign the above as my blueprint projectile using ConstructorHelpers in my Magazine class constructor so that I can spawn a projectile on firing a weapon.

I have tried using both FObjectFinder & FClassFinder but neither seem to work, and I am unsure of what I am doing wrong.

Here is from my Magazine.cpp constructor:

static ConstructorHelpers::FClassFinder<AFPWeaponsTestProjectile>
ProjectileClass = ProjectileBP.Class;

Note: This compiles but the ProjectileClass is NULL & as such never spawns. I have also modified this to use FObjectFinder as well as changed the type in the <>.

After doing some further testing I have solved the problem by adding _C to the end of the file path.