I don't believe that is correct. A Grand Jury can issue an indictment or not issue an indictment. But they don't convict or acquit. If additional evidence should come to light later, I believe that the same Grand Jury or a different one is completely free to indict.
Grand Jury proceedings do not provide protections for the accused. There isn't any cross-examination or opportunity for the accused to provide a defense.
I would expect a firearms instructor to recommend shouting at the loser to “GET OUT”. Make it loud enough for the neighbors to hear. It may get the perp to leave. If not, then it helps creates a defense against charges for the homeowner.