Start with slugging your bore and find out what ya got. Lead is quite sensitive to a proper fit in the bore. The rifling type shoots cast fine, while I don't know how to post a link, a recently read a articel on how the micro groove barrel actually gives more driving surface. Makes a lot of sense. I've not ever had trouble with cast in microgrooves beyond a good fitting bullet. The 1:38 twist prolly won't stabilize anything heavier than 300 grains, but will do better than ok with that weight. Far as I'm concerned, a 270 grianer is the best compromise of weight and speed; after that you throw weight but don't gain in energy performance over the bullets flight, so extra weight becomes a bad trade. The 1:20 twist comes from revolvers where they increase twist rate to compensate for the short barrel.
Lots have complained of poor performace with 44 Handi's, mine was not an exception. It shot jacketed quite well but I had to do a lot of homework and range work to get to 1600 fps with cast, mostly due to bore size.
