The Bisley Choke Gauge can be simply pushed down the muzzle when the segment fits the bore perfectly. Read of the gauge the amount of choke of each barrel. This high-quality accessory is essential when buying a fixed choke pre-owned item as the choke stamps may not correspond to the amount of choke in the bore - sometimes choke is bored out by a gunsmith and they do not always re-stamp the new choke on the barrels.