Bomb, Practice, BDU-33 (Dummy/Inert)
This is a Cold War/Vietnam War-era BDU-33 Practice Bomb, a 25-pound bomb with a spotting charge that releases a cloud of smoke on impact. The BDU-33 is used to simulate the MK 82 in low drag configuration. It is a small cast-iron and steel non-explosive ordnance that is used in training to simulate actual bombs. These training munitions, by design, have similar flight and delivery behavior to war shot munitions.
More can be read about the BDU-33 at Global Security.org. Measures ~25 inches long and ~4 inches diameter.