It's not an "artillery shell" (a projectile), but it does appear to be the brass casing for one, which held the propellant-powder charge. The baseview photo of your "apparent" casing shows your it is missing the primer-disc which was inserted into the center of an artillery projectile casing's base.
If it is an artillery projectile casing, its body will be thin-walled. Please post a photo of the narrow end of the object.
Also, please precisely measure the diameter of the opening in that end.
The base of artillery projectile casings typically has some kind of identification markings. You say there aren't any there -- but are you absolutely sure? Please clean the object's flat base more thoroughly, and check it again for markings, because some faint ones seem to be shown in the photo. I think I see an "A" at the 10-o'clock position... but maybe I'm just imagining it.
Edit, approximately 30 minutes after posting, for clarification:
Although I've been saying "artillery projectile," it might be an Indian Wars era 1"-caliber Gatling Gun (machine-gun) casing. That's anothe reason why super-precise measuring of the opening in the object's narrow end is needed. Also, there is no "extractor-groove" above the base-rim, so if the object is a projectile-casing it is likely to be from the latter third of the 1800s rather than a 20th-Century one.
