Penetrating gunshots are almost always fatal. However, since cranial fragmentation(and thus the recurring insult to the brain) can be limited if the gunshot came from smaller weapons such as pistols, I'd say survival could be a possibility. This especially applies if the damage is unilobar and non-fragmented(1).

Final mortality rate numbers are difficult to come by though, since not much research about this has been published. The prognosis is based on the patients initial Glasgow Coma Scale(GCS) number, a way of assessing the conciousness of a patient.


This table(1) is suggestive of mortality rates based on GCS-score. GCS is usually scored 1-12, but in this study they divided patients in A-D, with the D group being the largest and the one with the most patients(i.e most patients have a low GCS-score and thus a very high mortality rate). 59 patients were from Group A (22.3%), 32 from Group B (12%), 49 from Group C (18.5%), and 125 from Group D (47.2%).


Bihempspheric bullet trajectory. Both temporal lobes are affected. Severe, multifocal fragmentation.

In addition to the GCS-score, the degree of cranial fracture is also a determining factor in assessing the outcome of patients. Skull fracture will lead to fragmentation of bone into the brain, which severely lowers the prognosis. One study shows that shots to the head with either a 7.65 mm (pistol) or a ·38 special (revolver) results in severe cranial fracture in 82% of cases(2). These are small weapons compared to the rifles used in PR. The degree of tissue destruction depends, as you might expect, on the projectile used and the strength of the weapon(3).

That said, there are multiple case reports(though few studies) reporting surviving patients with little or no neurological symptoms post trauma. Most of these are suicide attempts, generally attempted with small arms(pistols or similar).

For a conclusive mortality rate I'd want to talk to army neurosurgeons from war zones, but I've no idea where these guys publishes their articles(if they do publish anything). However, what is certain is that it would be absolutely impossible to "revive", i.e operate, on a soldier shot in the head in the field. All patients shot in the head would suffer from cranial herniation, which would result in either temporary or permanent neurological symptoms involving cranial nerve affection and loss of conciousness. The fact that headshot soldiers in PR die instantly probably reflect both the fact that they would almost certainly die and the fact that even if they didn't, it'd be months or years until they could return to the battlefield.


References
1 - Roberto S. Martins M.D., *, M. G. Siqueira M.D.*, M. T. S. Santos M.D.*, N. Zanon-Collange M.D.* and O. J. S. Moraes M.D.Prognostic factors and treatment of penetrating gunshot wounds to the head. Trauma 2001. Department of Neurosurgery, Hospital Santa Marcelina, São Paulo, SP, Brazil

2 - P. Betz, a, , D. Stiefelb, R. Hausmanna and W. Eisenmengerc, Fractures at the base of the skull in gunshots to the head, Forensic Science Journal Volume 86, Issue 3, 5 May 1997, Pages 155-161.

3 - Constantin Stuehmer DDS, , , Katrin S. Blum MD, Horst Kokemueller MD, DDS‡, Frank Tavassol MD, DDS§, Kai Hendrik Bormann DDS, Nils-Claudius Gellrich MD, DDS and Martin Rücker MD, DDS, Influence of Different Types of Guns, Projectiles, and Propellants on Patterns of Injury to the Viscerocranium, Journal of Oral and Maxillofacial Surgery
Volume 67, Issue 4, April 2009, Pages 775-781.