Adds +3 to AC when used with a two-handed ranged weapon. It may not be used with any melee weapon nor with a one-handed ranged weapon.
Pern's Redoubt Helm
Adds +3 to Strength (thus, depending on prior Strength score, adding either +1/+2 To Hit in melee and either +1/+2 Damage with melee and thrown weapons) and +4 to Constitution (thus adding +2 to Fortitude saving throws and +2 to max HP per character level)
Adds +1 to Dexterity (thus, depending on prior Dexterity score, either 0/+1 To Hit with ranged attacks, either 0/+1 to AC, and either 0/+1 to Reflex saving throws)