An ideal ammeter is short circuit. A short circuit is a limiting approximation for a real ammeter, which will have some small (but not zero) resistance.
A short circuit is a link between any two nodes in the field of circuit analysis that compels the nodes to be at the same voltage.
As a result, an ideal short circuit will have no resistance and, as a result, no voltage drop across the connection.
Therefore, short circuit will have zero resistance.