micrometer or vernier calipers ......(calipers would be best because they allow you to check internal dimensions as well) a good set will put you down at least $65..... ideally you want the barrel to be anyware from .001 and no more that .005 thousandth's oversize for a tight interference fit. .... as long it isnt to far off you can have the barrel knurled to tighen up to .025