Sighting In Mossberg Slug Gun

I've recently ran into some unexpected trouble sighting in my Mossberg 500 for slugs. The Mossberg 500 combo comes with a field (smooth bore) barrel, and a rifled barrel for slugs. The rifled barrel is equipped with scope mounts.

The riffled barrel is capable of shooting sabot slugs accurately up to 100 yards. Be warned that it could likely take several boxes of slugs to get a tight group at this distance. I came with 4 boxes (5 slugs per box), and was barely able to get the gun sighted in at 50 yards. I've still got some work to do because it's still shooting a few inches low. Right now it's ok, but not perfect. I ran out of slugs, so I'll have to finish up on another day. Keep in mind that a box of 5 slugs can be about $13, so this will end up being an expensive exercise for me.

If I were to do it all over again, I would find some way to try to do some initial bore sighting before actually firing. Unlike a bolt action rifle in which the bolt can be removed for relatively easy bore sighting, there is no way to look down the bore of the shot gun while attached to the stock. Therefore, the only way to bore sight the Mossberg 500 is to remove the barrel from the rest of the gun. This can make fixturing difficult for bore sighting, since a standard gun vice will not hold just the barrel. I suggest using some sand bags to hold the barrel, while the scope and bore are coordinated. If I had done this to start I likely would have saved some ammo.