There was a magic bullet
SPOILERS*****SPOILERS*****SPOILERS
In answer to the two-year old post below, yes, there was a magic bullet depicted in the plot, towards the movie's climax.
Robert Culp removes a tiny portion of the broken, enchanted gold seal to fabricate something of a 'holy' bullet that he loads into an old-fashioned, muzzle-loading, black-powder pistol.
The broken gold seal was actually a magical 'ward' to seal in an ancient underground chamber used in ancient druidic but evil occult practices. In the movie, even half of the gold seal was powerful enough to hold at bay a human occult practitioner who had morphed into a cross between an abominable snowman and werewolf. He actually looked like the rakshasha demon in the 1974 TV-series, 'Kolchak: The Night Stalker', episode, "Horror In the Heights".
The GOLD bullet was something of a, 'mcguffin', a plot device used to advance the story, but of itself not important. In occult mythology we usually associate SILVER with holy bullets against creatures such as werewolves. Silver bullets are supposed to work equally against the undead such as vampires if shot through the heart. It's not fully explained why gold would be an effective holy weapon against an arch-devil. There is only the association that the gold came from a holy, enchanted seal which made the gold supernaturally effective.
In reality, GOLD would be the most deadly and effective bullet projectile against a living being. Gold is the softest, densest metal (not counting mercury). A gold bullet would cause significant damage, striking a target with more force and energy and flattening more efficiently than lead. A gold bullet would truly mess up a person big time. It would be akin to using a dum dum bullet on someone. Against the physical materialization of an arch-devil, the holy gold bullet could not kill such an entity but cause it enough pain to delay it, as Robert Culp intended. The arch-devil in SPECTRE looked almost exactly like the intelligent reptilian alien, the gorn, in STAR TREK's episode, "Balance of Terror".