Found two of these in a field today. It is about 8 mm wide and 18 mm long. They appear to have been fired, but in the center there seems to be cement. Thanks for the help
I can say that 8 mm is close to a .32 caliber. The evidence is that it is a jacketed bullet that was fired from a rifled firearm (most likely a pistol), and it looks like it was fired into the ground. You'll need to mic the diameter to make a closer judgement.
That's a rifle bullet. 8mm was a popular around 50 years ago when there were a lot of 8mm mausers converted into deer rifles in hunter's hands. .30 caliber is 7.62mm, so depending on how you measured, it could be a .30 caliber from a number of possible cartridges (.308, 30-06, 30-30, etc...)
Not an expert, but I tend to agree with "cheese", the length of the slug indicates it was probably to long to be fired from a pistol or handgun, just my opinion.