Shotgun powders are usable in any gauge "IF" the load is proportional to the gauge. The reason many powders have data only for 12 ga is they are normally a fast powder suitable for light loads. Many people are loading Down the 12 but not many are doing so with the smaller gauges. Most are Hot-Rodding the 20 &, etc trying to make a 12 out of it.
You can for instance load Red-Dot in a .410 "IF" you put an appropriate amount of it behind about 5/16 oz, It, of course, will not be suitable behind 11/16 or 3/4 oz. Nothing at all magical about any bore size, equivalent loads in any gauge are proportional to the square of the bore's diameter. 20 gauge loads proportional to a 12 are 71% of the 12s, while those of the .410 are .32%
