I just made a lengthy thread about this. Now, this is my finding with 44 magnum brass, but I'm guessing it would apply to anything similar in a Lee FCD.

Loading .430 lead projectiles in thicker walled brass, the Lee will smash them down to .429-.428". By "thicker" I mean some of my brass has a thickness of .010", and others .011".

.011" thickness brass and a .430" projectile, the Lee die will 100% take the bullet down to at least .429" and in my Ruger SR, that means leading.

It is super easy to tell when this is happening as it takes considerably more force to run the round in the die than if it is a smaller size.