All trunnions ship with 2 rivets to be used in securing the trunnion bracket
Rivet style trunnions come with additional "top" rivet that is custom machined to fit within the recessed pocket within the bore of the trunnion
(to give clearance for the barrel to pass though)
Here is a picture of a standard "Lipped" barrel (used with long face trunnions) verses a "Lip-less" barrel
Here are the four styles currently offered:
Long Face Rivet Style: This is the standard trunnion used with a standard "Lipped" barrel and is drilled to be riveted to the upper receiver
Long Face Weld in Style: These are for weld together builds, they are faced for a standard "lipped" barrel and have no provision for a top rivet
Short Face Rivet Style: This is a rivet in trunnion designed to be used with a barrel that has been profiled without a lip. The standard PPS barrel has a .160 lip on the chamber end of it, the face of this trunnion has been set back .160 to allow for a smooth barrel to be set flush with the trunnion face and pinned in place.
