On the few occasions I've modified chokes myself they were all fixed chokes on inexpensive guns. I either used a round rasp to "open" the side of the choke opposite from where it was shooting on double guns, or on single-barrels sawed the barrel off slightly to eliminate some of the choke--which often fixed the problem.
Have you ever tried it on a gun with chromed bore/s? I think that these Stoegers fit that category.
