The bolt weapon carried by the Daemonhunter has a magazine of psychically impregnated psycannon bolts. These vary in calibre from bolt pistol size to storm bolter shells, although their potency is remarkable in all cases. A bolt weapon (storm bolter, bolter, combi-bolter or bolt pistol) with this upgrade counts as AP4. Only Armour saves may be taken against psycannon bolts; Invulnerable saves may not be taken.
Psycannon Bolts
Source: Codex: Daemon Hunters, p. 18