Unless you have a tight bore, that is going to lead badly at most vel. You want a bullet at least 0.001" larger than groove dia. Measure the cyl throats, if they are larger, then match that, if smaller, then you need to open the throats for best results. With a slow powder, 1200fps shouldn't be a serious leading issue w/ good bullet fit. If you are NOT going to slug the bbl or cyl, buy them at least 0.430" in dia, even 0.431" will likely shoot better.