Hi, guys,
I have to disagree with Grump. "In battery" does not necessarily mean locked, and blowback actions definitely go into battery when they close. They too, have some means, such as a disconnector, to prevent firing if the bolt/slide is not closed.
Bwana John is correct in that advanced primer ignition sets off the primer while the bolt still has significant forward momentum, but the bolt is in battery by the time the gas pressure kicks in. API is what allows open bolt SMG's to operate with lighter bolts and lighter springs than are required by guns firing from closed bolts. (Example: the Thompson SMG can be cocked with the little finger, while the semi-auto (Kahr) version requires two men and three small boys to haul the bolt back.) But BATFE says open bolt guns, even semi-autos, are too easily converted to machine guns (true) and won't let them be produced.
Jim