Double-barreled sleeves are perfect for crimping over wire and monofilament to create your own leaders to use when fishing for toothy critters. The copper sleeves have a smooth, burr-free finish that wont damage your line. These sleeves are perfect for making end loops in the leader material. Thin-walled sleeves are ideal for mono or fluorocarbon leaders as well as wire. Per 50. Cancer and Reproductive Harm- www.P65Warnings.ca.gov. Type: Leader Sleeves.