Turns out that Monty Python got it right. According to Karl Steel there really was a Book of Armaments....

Bless the rifle and cannonball, and let the above blessing of the arms [be done] with all the appropriate changes, and let them be sprinkled with holy water. Let the gunpowder [pulveris tormentarii] be blessed, and the projectiles: namely, the bullets, whether of lead or iron, whether collectively or individually.

It was the 18th century -- rather post-Arthurian. Still, close enough.