The “break” signal (Ctrl-C for many computers) stops the solver and returns control to AMPL, but does not necessarily return any solution from the solver. Some solvers may make provision to “catch” the break signal and return the best solution found so far; consult the solver directive instructions to see if this is the case for the solver that you are using.
Various solver directives can stop a run and return the current incumbent optimal solution, but they must be given before the run begins. Available stopping criteria may include number of branch-and-bound nodes, number of integer solutions, and total time. Directives are different for each solver, so consult AMPL’s solver directive instructions for details.