You must be forgetting that sometime around 1630-60, at least in England, according to C.H.Firth, Cromwell's Army (1972 4th ed.) p. 80, the musket barrel was cut down from 4 feet to 3 feet. But in any case, she wouldn't have had to actually reach the trigger with her own hand. Presumably, a British soldier's finger was already at the trigger, else the entire "barrel at her breast" exercise would have been useless. If the musket were actually "bound beneath her breast" she would only have had to slowly lean forward, and then swiftly lean back, to utilize the soldier's ready hand as the instrument of firing.

"Her finger moved," you object? Poetic license, say I!