Remington Model 740 - Produced from 1955 to 1959. Chambered in .244 Rem., .280 Rem., .30-06 Springfield, and .308 Win.
Remington Model 742 - Produced from 1960 to 1980. Chambered in 6mm Rem., .243 Win., .280 Rem.(marked 7mm Express 1979-1980), .30-06 Springfield, and .308 Win.
Remington Model 7400 - Produced 1981 to date. Chambered in 6mm Rem., .243 Win., .270 Win., 7mm Express, .280 Rem., .30-06 Springfield, .308 Win., and .35 Whelen.
All standand versions have a 22" barrel with open sights and a 4-shot detachable magazine. Some internal improvements have been made over the years but the basic gas-operated action remains the same. Other than cosmetic changes and various grade versions, the 740/742/7400 remains pretty much the same rifle. Hope this helps!
Steve Mace