This is a special hollow projectile designed for shock testing of fuze boosters. Judging by the cavity size, the boosters should belong to fuzes used in 20-mm or similar calibers.
The projectile is made from lathe-turned brass. It is cylindrical, has a driving band, is sligtly tapered at the “ogive” and has a screw-on base cap.
The measurements are:
Length = 28,9 mm
Diameter at the body = 10,39 mm
Diameter at the driving band = 10,66 mm
Weight = 16,3 g
The booster to be tested was put inside this projectile, the cap was screwed, and the projectile was loaded in a 10,4 x 47 mm Vetterli-Vitali case. It was then fired in a Vetterli-Vitali single shot rifle against a backstop consisting of plywood boxes filled with cotton, to avoid deformation of the fired projectile.
The Vetterli-Vitali rifle and its ammunition were remnants of the spanish civil war of 1936-39. The cartridges I have seen were made between 1891 and 1916 and had brass jacketed bullets. It seems that this caliber was chosen because it permitted the use of a large enough projectile.
I have no information about the propellant weight or type. Perhaps they used the original Vetterli loading after pulling the Vetterli bullet and putting this projectile, but the projectile seems too long for that. Maybe they used a reduced load of smokeless rifle powder, or maybe black powder, as I have seen fired projectiles and they were stained black.
These experiments were made in the '70s at the Polígono de Experiencias de Carabanchel (Carabanchel proving ground). Similar tests were carried out later over the years at the Palencia factory of Santa Bárbara, using steel projectiles and 20 x 110 Hispano-Suiza cases.