What has a bullet 3.5" x 1.25" ?

[Springfield]

With a steel/iron body and a lead tip, with a copper sealing ring?

[pworley1]

That looks like a 30mm canon round.

[GhostHawk]

Offhand I'd have to say 30mm auto cannon.

[salpal48]

Looks like a 37MM Hotchkiss Revolving Cannon projectile or
The Lite Hotchkiss Mountain Cannon projectile 1.65"

[samari46]

37mm from about 1890's through WWI and up to WWII. Have a few like that but no lead tip. Mine have a fuze screwed on the nose
all deactivated. No powder in projectile. Frank

[Springfield]

After cleaning it up I see it says 30mm on the copper ring.