An apparatus comprising a cartridge loop, the cartridge loop having a coupling interface with an opening defined in part by a pair of substantially parallel lines separated by a distance; and a coupling comprising first and second ends and a link that connects the first and second ends, the first end connecting with the coupling interface of the cartridge loop wherein a portion of the link adjacent the first end has a first thickness that is greater than the distance between the substantially parallel lines and a second thickness that is less than the distance between the substantially parallel lines.