If you’re using sabots, then the sabot is what the rifling acts upon. If the bullet is molded concentricly ie no voids or contamination etc. and it fits snugly and squarely it should at least group to some degree. If you mean that the bullet is being loaded over the powder charge then yes it needs the base of the bullet to obturate, bump up, to fill the lands. The tabs as you call them look like the lead base has flowed into the slits or spaces between the sabot petals not into the lands. In summary if the sabots are the correct type recommend for the application and powder charge range for bullet weight and the bullet seats squarely and snugly in the sabot and the bullet is well cast it should shoot at least fair. I’ve shot all kinds of jacketed and cast bullets loaded in sabots out of my 50 and 54 M loaders with very good accuracy. Keep tinkering safely and good luck next season.

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