SELECT "model" FROM table_203_253 ORDER BY "caliber(s)" LIMIT 1
which model is the only model to have the lowest caliber ?
CREATE TABLE table_203_253 (
    id number,
    "model" text,
    "frame" text,
    "years mfg'd" text,
    "caliber(s)" text,
    "production" text,
    "barrel" text,
    "notes" text
)