the m49a1 sub-caliber device, which uses nato 7.62 mm rounds, is typically used for zeroing the sight and for qualifying purposes. the sight would be visually zeroed by setting crossed strings over the gun barrel opening (there was a rubber loop and notches at the end of the barrel to facilitate holding the string in the correct position), then looking through the barrel of the sub-caliber device (and crossed strings) and setting the fixed stadia, reticle sight to the same target. the sub-caliber rounds could then be used to further refine the zero and to qualify.