Vietnam War 20mm Practice Shell -
The 20 mm caliber is a specific size of cannon or autocannon ammunition, commonly the smallest caliber which is unambiguously a cannon (or more commonly today, autocannon) and not a heavy machine gun.

There are few weapons (aside from shotguns and large game hunting rifles) which have been built which fire projectiles between .50 caliber (0.50 inch/12.7 mm, roughly 12 mm caliber) and 20 mm caliber, though the 14.5 mm caliber was used by some Soviet machineguns such as the KPV and antitank rifles such as PTRS, PTRD, and NTW-20.

This is a practice round. All rubber construction. White color with black letters stenciled on the top.