The pellet thing may work assuming that you can get a pellet that fits in the bore correctly and you can fire it using what I could only assume would be from a primed empty case. Seems like a bigger PITA than just looking thru the bore at a distant light source and matching the scope to it.

We sight in rifles all the time for long range prairie dog shooting and with good equipment and ammo it takes less than 8 rounds to get them 'on'. We sight 'em thru the bore the "old" way. one shot at 25 yds. and make corrections for 100 yd. x4, fine tune them for distance and call it a day.