Background: We investigated interoperability for a first aid provider to perform simulated use of three tourniquet models of maximal, moderate, and minimal familiarity. Methods: The experiment was focused on the tourniquets used by an expert who rendered aid on a manikin by using three models of tourniquet with different extents of familiarity: The familiarity with Combat Application Tourniquet (C-A-T) was maximal; that for Special Operations Forces Tactical Tourniquet (SOFTT) was moderate, and that for Military Emergency Tourniquet (MET) was minimal. Each model had a band-and-rod design. Interoperability changes as intermodel differences were beneficial or costly in that performances were improved or impaired in units of time, ease, blood, and pressure. Each model had 10 tests, and test order was randomized by model. The HapMed Leg Tourniquet Trainer simulated a limb amputation. Results: In comparison of interoperability burdens, sums of 10 test durations by model for C-A-T, SOFTT, and MET were 38, 77, and 64 minutes, respectively; C-A-T was fastest (p ≤ .002, both). The sums of times to stop bleeding for C-A-T, SOFTT, and MET were 334, 953, and 826 seconds, respectively; C-A-T was fastest (p ≤ .0013, both). The sums of blood losses for C-A-T, SOFTT, and MET were 2105, 3287, and 4256mL, respectively; that for C-A-T was least (p ≤ .0005, both). The mean ease of use differed, with C-A-T being easiest (p ≤ .0046, both). The mean pressure differed, with C-A-T being higher than SOFTT (p = .0073). Conclusions: Timesaving strongly favored the model with which the user had maximal familiarity. In theory and simulation, interoperability bears costs in successfully attaining it, in maintaining it, and in failing either. The user's familiarity with tourniquet model was associated with improved interoperability as seen by improved performances. If multiple models are fielded, then organizations may plan on extra spending, supplying, training, and managing.