[hondo68]

What's the advantage of having a 12ga slug (or sabot) rattling around in a 10ga overbore? It seems that the blow-by/leakage would negate any advantage of a 3.5" shell.

[Dave McCracken]

Hondo, the theory is the sabot or wad obturates to fill the larger bore. One of the blessings of plastics technology is this.

Mostly, the overbores are designed to improve patterns and performance of shot loads, not slugs.
They also offer a bit of recoil reduction, tho this is overstated by the makers.It does help a bit with the barnburner loads, but I doubt one could tell the difference with most field ammo.

HTH...