There doesnt seem to be any hard and fast rules regarding this subject. In very small cases, it seems that I primer alone will almost always push a bullet from the case. My thought is that smaller cases experience higher pressure than larger ones with just a primer pop due to their smaller volume. I have only loaded a "squib" once. It was a 7x57 and the bullet did not dislodge from the case. I have seen squibs in pistol rounds that nearly cleared the barrel. I saw one instance where a .45 in a 1911 came so close to clearing the barrel that half of the bullet was protruding from the barrel. It was a funny sight to see.