It sounded odd to me also, but 2 Barrel Clips came with the kit, the care package contains 4, 2+4=6. When I talked to Ken Scott at Vans I was told we need 6. When I ordered my missing parts today, Gus said I needed 4 more clips. I'm just going by what they told me.
However, I don't see any more "barrels" in the plans we have so far, so I dunno. Don't shoot me I'm only the messenger.