"And it wont drop as much neither and wont lose as much velosity compaired In theory as a 12 will"

Not correct on both accounts....a 20ga will drop,lose speed, and energy, quicker than an equivelent 12ga.

Here is the data for one of the best rifled sabots available.....

Index No. Velocity (fps) Energy (ft-lb) Drop (in.)
Muz. 50 100 Muz. 50 100 50 100 150
12.... 1900 1770 1648 3086 2682 2325 1.8 2.4 0.0â€*
20.... 1900 1750 1615 2084 1774 1506 2.0 2.7 0.0â€*