Allowing for the fact you may not be using a vernier I would hazard a guess at a 9 x 19mm luger/para bullet! Or poss .380?
Mark, part of the challenge of ordenance/detecting is doing a bit of research yourself. Please don`t get me wrong or feel put off but the internet is a powerfull tool (as is this site - within its own/our confines).
Part of the mystery of detecting is then doing the "detective" bit after.
Doing a bit of Miss marple will make your finds more interesting/challenging. Where did you find it? Is it a known military area, does it have history (battles, training etc), is there any local folklore, etc?
As to the item, things such as condition (rust, verdigris), dimensions, any markings, rifling marke, more pics, types of metals all go towards id`ing the item.
Keep up the detecting & good luck (remeber you`ll learn more from learning it for yourself - & us old soaks are NOT always right!!).