I will post more pics and the exact dimensions soon. THanks for all the feedback. For what it's worth, I sent the info to a firearms expert and here's what he sent back, but he couldn't see the photo--
I couldn't get logged in to see the picture... But I am a bit perplexed from the descriptions. The US grenade launchers are 40mm, so you could measure and see - but I can't imagine it came from those as these normally fired exploding munitions or clusters of shotgun pellets or (rarely) flechettes. I suppose some homemade thing might be seen, but I seriously doubt that it was fired from a modern grenade launcher.
Rifled weapons were used by the colonists during the Revolutionary war, one reason they had a greater range on the British who were generally armed with smooth-bores, as were some of the Americans.
I am not an expert on older weapons, but some had pretty large bores. And even though shot was often fired from larger bores, shooters often made their own balls and shot so it is always possible that you could find some oddity that was created by some experimenter. (Though, again, this would be exceedingly rare).
I am not certain (but pretty)... but I don't think rifled bores were seen in cannons and such until much later. Yet these seems to be from a larger rifled artillery piece or some odd-ball shotgun. That said, I have to wonder (also given the less weathered look it apparently has) if the projectile is newer than the Revolutionary War -- perhaps Civil War or later vintage?
A mystery, and I'm sorry I can't help you more.