An atm long/short calendar will perform inside/outside the atm straddle range, or approx a sigma, provided there is a flat term-structure of volatility and no stock-specific vol anomalies. Of course, most traders treat them as binary event wagers.
ATM long calendars are generally a poor-proposition in a tight range as you're winning on gamma but losing on vega. You've bought the position at best-case, so I'd rather trade them directionally, as hedges, or neutral with some term-structure edge.