Found it.
The Hornet cartridge for the Savage 10/11/12/16 rifle.
5.7 x 28mm FN
Looks like a miniature 221 Fireball.
Case length is 1.135 inches. COAL is 1.525 inches.
Holds about 7.5 grains of powder.
From a pistol barrel (4-3/4 inch) gives about 1,700 fps with a 40-grain bullet.
From a rifle barrel ought to get above 2,000 fps.
.