Powder is Not Gauge specific, it is load density specific. Load density = the weight of shot lifted per Sq In of bore area. Considering nominal sizes a 28 has a bore diameter of .550" & an 8 gauge a diameter of .835". As the area of a circle is proportional to the Sq of their diameters an 8 gauge has 2.3 times the area of a 28. "IF" you loaded the 28 with 3/4 oz & the 8 with 1 3/4 oz they would use the same powder. All other gauges would have a proportional load for that powder. You could even load the .410 with it if you dropped the shot load in proportion to that little bitty hole. You would have to go down to between 3/8 & 7/16 oz in the .410. Nothing magical at all about it just fairly simple math. The same can be done with any powder, different powders are of course suitable for different load densities.
By dropping the charge weight & loading on the Low Velocity principal a faster powder can be used than would normally be used for that shot weight.
