Got out for a bit between rain showers today and found these. I'm fairly certain they're modern. I know nothing about this stuff. They measure just over 1/2 inch. I only ask because they were found on my property and we don't own any guns.
The "modern day" 9mm parabellum has been produced since 1902 and .45 ACP since 1904, which I am near certain have been around longer than you have. : ) haha
you can tell by the tarnish on the bullets jacket and the oxide on the lead on the base those were in the ground for quite awhile....don't know how long you've owned the property but probably someone shooting targets before you got there.