Same here, but this is an air rifle. My beloved Diana 54 Air King.
@Regen: I don't think I can deliver the message that well. Lol. I can manage a stone face, but fake sympathy always ends up with me laughing.
LOL. Me too.
In this case, if it were me, I would either say nothing and make use of the chicken in some way or other. For example make them my own. My garden, my chicken.
I wouldn't kill them.
And in case she notices and wants her back, I could either say: these are mine, I found them in my garden. Or: You can take them back, but only on the condition that you never allow them into my garden again.
Or storm over to her, red faced and spitting to yell at her to keep her chicken where they belong, or else... But this I would have to do at once or my anger wouldn't be so convincing any more.
Or after calming down I would go over to her and put on my serious face and tell her in a rational tone, what happened, and that I hoped this was just a single incident and not on purpose. Because otherwise I would consider it a unforgivable rudeness and that would be a pity wouldn't it? After that I would put on my most malicious smile and walk away without giving her the chance to answer.
The last one would be my best solution, but I have trouble not to tremble with rage when saying something like that. But maybe being considered crazy would be to your advantage in future dealings with her. LOL
In reality I think I would take option one.
LOL: about rifles: my father actually shot at chicken and stray cats with his air pistol. Several times. It doesn't kill the cats, the bullets never stuck, but it hurts and tought them not to come back. So he tried it with chicken, too. He hit one in the butt and it died. Oops. So we had chicken for dinner. One of the neighbours told him a few days later, that he had some chicken missing and we should watch out for martens. My father thanked him gravely for the warning.