I shoot upland with side by sides only these days but have used over/unders and autoloaders in the past. My side by sides have had barrels of 25", 26", 27", 28" and 30" and all will work fine for me if the balance is right. For grouse 25-28" works fine for me, neutral balance on a twelve ga, progressively more weight forward as the gauge gets smaller, gun lighter and barrels shorter. For more open country for sharptails, chukars and pheasant longer 28-30" barrels work best for me. My current favourite all round upland gun is a Westley Richards 12 ga hammergun from 1873 with 30" barrels. Fits like it was made for me and truly one of the finest handling guns I have ever used. Barrel length never even noticed.