Attach the BSA boresighter to the muzzle, line up the cross-hairs with the grid lines(20*20 grids) and you're done with no shots fired! Will help you get on paper without wasting shots. Comes with .177 arbor, .22 arbor, 6mm arbor, .25 arbor, 6.5mm arbor, .27 arbor, 7mm arbor, .30 arbor, .32 arbor, .338 arbor, .35 arbor, .375 arbor, .44 arbor, .45 arbor, .50 arbor. All packages in heavy duty hard case. Also, fits shotgun gauges 10, 12, 16, 20, 28. Will NOT FIT .410 shotguns. This product CAN NOT be used on rifles with a compensation/muzzle break. Product Dimensions: 28*21*7cm ; 1140 g Boxed-product Weight: 1250g