Bullet prop tail nut for 1/4 inch shafts. 
Not actually a nut, but has a 1/4 inch hole with a set screw. 
Hole depth is a little over 1/2 inch. 
Bullet is about an inch long and about 1/2 inch diameter. 
Made from stainless steel. 
Not recommended to hold the prop in place unless you file a flat spot under the set screw.
Even then, putting this behind a lock nut would be better. 
Includes a bullet nut and set screw.