Case diameter is about 5/16" and 1.25" OAL. The bullet end is sized for a 6mm. Headstamp is illegible, looks like it was used a lot. I got it with a bag of 250 45 ACP cases that were from a range in central Texas.It has a very distinct shoulder. Tiniest center fire rifle round I have ever seen. Might be useful for squirrel or mice.