Various Designs for Anti-penetration Devices (APDs) It is vitally important that you consult your local Kingdom rules before using these designs. We cannot be held responsible for your ordering something that you are not allowed to use. Asgard APD - $ .65 each IMPORTANT - Because of the angle of the built-in nock, this APD is only usable by right-handed archers. This does not apply to crossbow users who will be cutting the nock off. Installation of Asgard APDs on arrows and crossbow bolts Siloflex Tube APD - $ .45 each The flight characteristics of this APD are rather impressive. Made from 160 PSI Siloflex tubing these APDs have a groove for seating against the bolt shaft and have rounded edges to prevent abrasion of bare skin. They are virtually indestructible! Installation of Siloflex APDs on crossbow bolts Installation of Siloflex APDs on arrows with nocks