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.

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