I think you absolutely have a legitimate concern. From your initial post, I'm taking it that this has happened more than once.....which indicates it will probably happen again. There are no rules or ballistic formulas to indicate what a ricochet will do, or where it will go.
I would begin documenting everything, even when you hear shooting and DON'T hear a ricochet. Not to try and make them quit shooting, as they have every right to enjoy their property, but just as protection for yourself, and maybe another neighbor also....maybe the property owners on these folks' other side are catching a stray round or two themselves.

I would approach them again, when you hear a ricochet come onto your place, and just explain about your documentation and your concerns. Be nice. Maybe invite them back over to the spot where you heard the stray round, and see if, together, you could come up with a scenario that might explain why it's happening.

Definitely get back on the local law enforcements a$$.....his response was unacceptable in my opinion. Let him or her know about your documentation, including speaking with him or her, and suggest you may be contacting the State Police if you have any further incidents.

"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.