both 12 and 20 a bull barrels on USH...and thats waht you want...NEVER NEVER have to even think about barrel heat during sighting in..just too thick for it to effect....20g is 12g barrel bored out for 20g and 12 gauge is a 10g barrel bored out to a 12g...dont dick around with non USH....20g USH--dats da ticket!