It’d really depend on what the head is being shot with With rounds like .22 LR, .38 special, or an arrow, it would make sense that enough tissue is left intact or at least in usable reach of the slime to facilitate repairs. However, something like a .45 ACP, a rifle shot, or a shotgun loaded with anything heavier than birdshot is quite likely to wreck the head and brain beyond repair. With higher-powered rounds, it becomes less of a question of how much damage it does as opposed to how much is left afterwards, with the rest painting the landscape as a fine red mist.
At first glance this appears to be reasonably well-modeled in the dev builds, though I have not had a chance to test anything more than a 9mm pistol and a shotgun.
Slightly off-topic, but the differences in real life in soft-tissue damage when you compare standard military surplus 7.62x54mm rifle ammunition with commercial soft-point rounds of the same caliber is quite striking. I tested this several years ago using a Mosin-Nagant and two watermelons at a range of around twenty five meters. With the military surplus round, it punched a relatively clean entry wound and created a nasty crater of an exit wound, after which the bullet was clearly visible ricocheting off the ground behind the target. With the soft-point round, the watermelon simply ceased to exist. Upon closer inspection, some pieces were flung six or seven meters in all directions and the scent of watermelon lingered in the air for several minutes in about a ten-yard radius.
Simply put, not only does the caliber matter but the composition of the bullet itself. While a military surplus 7.62x54mm rifle round is easily capable of doing lethal damage to something similar to a human, a lead-core soft point will blast it into a fine pulp and decorate the surrounding area.
What this really boils down to is “More ammo types, please!”
