Scrape an area to make sure it's metal. Use a magnet to see if any (core?) is iron. If solid, do specific gravity to see what metal and how heavy.... most likely lead with copper "skin". Using a digital calipers (I'm sure you have one, snicker) measure length and width in metric (cm/mm). May be 7.62 mm in width if most common rifles or even 6.5. The different base suggests it may have been a tracer... they hollow the base and fill with substance, usually magnesium, to get the base to give off bright light. Match your findings to available bullet identification tables. If not metal, start over with another approach (tsk,tsk). ╦╦C