Have to disagree steif. Inside lubed bullet. The wide grove with the marks are for the bullet lube while the small circle grove just above it is the crimping groove. That would put the lube inside the case. Hollow base? Can't tell. I would guess it is cast for about a .32 Colt revolver bullet? Not sure without measurements. Colt made a .32 Short Colt and .32 Long Colt both and I think they both used the same bullet. There was also a .32 S&W Long and a .32-20. But I think the 32-20 used a heavier bullet with 2 grease grooves as it was also used in a rifle. If the bullet is .32 caliber this data is good if it is smaller then I can't say for sure. Need some measurements and weight if possible. I don't see any rifling marks either but the bullet looks fired. Colt had a left hand twist and S&W a right hand twist if I'm not mistaken. All of this info is just guessing from the limited information available. Modern design, meaning it is after smokeless powder became available. Dang, I hate guessing. Monty