Half a dozen independent barrier operations comprised the SEALORDS initiative. The intent was to block a Viet Cong supply flow estimated at 175 to 200 tons per month. The Americans had great success, but their overall impact remained difficult to gauge. In one of the operation's deemed most successful, Giant Slingshot-an attempt to isolate the Parrot's Beak section of Cambodia, phased out in May 1970 after the invasion of that country, an enormous effort was made by fifty or more PBRs. A typical sailor assigned here logged 172 ambush missions during his Vietnam tour. Total seizures in Giant Slingshot came to 150 tons of ammunitions and 400 more of other material, compared to an estimated Viet Cong supply flow (November 1968 to May 1970) of 3,300 to 3,800 tons. Lt. John F. Kerry, a Swift boat commander at Phu Quoc Island in 1968-69, recalls, "You'd randomly stop a boat among the dozens that were going up and down. But for the one or two you stopped, hundreds of others went by, and you knew weapons were slipping by you."