I have a 740 and three 742s in different calibers. The 742 has a slightly improved bolt and a much better stock with a higher comb for use with a scope.
My 740 is in '06. It sometimes does not completely close the bolt on a manual charge. I always push the bolt handle forward when first charging the chamber (like using the forward assist on an AR). It cycles fine from the gas system for subsequent shots. Maybe the spring is weak or worn locking lugs. After all its only about 50 years old and been used a lot. Otherwise, runs perfectly.
My 742s are two ADLs in .308 and .280. Both run flawlessly. The third is a BDL '06. It has an upgraded stock with nicer wood and a full cheekpiece. Otherwise, it is the same as the ADLs, and is equally reliable. All shoot quarter size groups at 100 yards with decent ammo. About 1.5 MOA.
They are somewhat sensitive to handloads. You must use small base dies if you reload, and it is best not to exceed factory load data equivalents. Get too hot and extraction is too violent and cases get torn at the groove around the case head because the case is pulled from the chamber while still expanded. This only happens with handloads worked up with my hotter bolt gun recipies. They are all fine with factory ammo and more "moderate" handloads.
The 7400 also has some slight bolt change. I really don't know much about the 750.
If you get any one of these, keep the chamber clean, not just the bore, and it will run great. People who report problems almost always see them disappear after using a chamber brush. It has brass bristles, chamber size, on a z shaped twisted wire handle to gain access through the ejection port, as the bolt must be locked back for cleaning and can't me removed from tbe back of the receiver like with a bolt action. Keep the chamber clean and they run fine.