Understanding calibre
In most cases, the distance between opposite lands inside the barrel gives us the calibre of the rifle. In the image, the distance between the lands is 243/1000ths of an inch so this is a .243 calibre firearm. Knowing this, we can purchase the correct ammunition for a firearm.
![Firearm Bore](https://ke-courses-production.s3.amazonaws.com/asset_files/production/4561/attachments/original/HEX-bores.jpg?1592422845)