A simple model is presented which allows accurate prediction of bias margins of gap-tolerant half-disk propagation tracks for bubble domains. After this is verified by comparison with experimental margin data, an "isomargin" plot is derived to show how the margin varies as a function of and , where is the minimum linewidth and is the inter-bar gap. The bias margin is shown to decrease along a fairly straight line which goes to zero when equals the runout diameter, i.e., when , where Wsis the bubble stripwidth or average bubble diameter. This agrees with experiment, and means that the minimum resolvable feature for half-disk type patterns must be less than , and probably will not be much larger than to . It is concluded that, if made with perfect Permalloy, T-bars and half-disks should propagate isolated bubbles equally well. The advantages of half-disks over T-bars are 1) the fatal bar-crossing problem of T-bars with multiple bubbles is avoided, 2) the minimum propagation field is lower than for T-bars, and 3) half-disks seem more tolerant of "bad" (e.g., high-coercivity) Permalloy. Also tabulated are the effects on margins of variations in the device parameters of a representative design, as might be encountered in a fabrication process with finite tolerances. A brief discussion of stop-start margins is given in conclusion.