Fast Forward - Expansion
Examining each poorly hollowed-out skull in the table, I believe we've seen such growth before—not just in size but also density, at least some of them. While it was true that it would need a bit more oomph to get through to the thicker and denser ones, unless they could completely deflect bullets— much less a .22 caliber round, we wouldn't worry too much about it at the current moment.
But yeah, while there were anomalies like Hulkers or a type of Stitcher that could take more than one shot to the head either by its sheer physicality or ability to "equip" more heads, the vast majority of our enemies would drop down with a simple nick to its weak point. On another note, a hollowed one—or a coconut, new working title—had already had its head punctured so its ability to take bullets to the head doesn't count.