Daryl,
Among others, one of the reasons to use a sabot is to reduce friction (pressure) between slug and barrel, especially when using other than soft lead slugs. Another reason is to "adjust" free space between slug and barrel, since bore diameter of shotgun barrels vary quite a lot, especially smoothbore ones.
In the brass slugs we make, sabot is mandatory and less friction results in higher velocity, as well.
Lefteris.
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