Hi mate your gunshop should be able to do it for a small fee mine charges £2.
This is a chronograph pal http://www.jsramsbottom.com/products/chronographs/cb625-combro-cb625-air-rifle-chronograph-mk-iv-model.html
And to clarify you want to check he power ourput of your rifle (fperlb)
Atb
Nicj