Bushmaster ACR Upper Trunnion Plunger Pin, .156x1.38. This steel constructed .156x1.38 upper trunnion plunger pin is manufactured for the Bushmaster ACR rifles chambered in .223 Rem and 5.56x45mm. The upper trunnion plunger pin is responsible for retaining the upper trunnion plunger within the rifle and is perfect as a spare on the workbench or as an immediate replacement for a lost or damaged part.