Ed the two 12 bores that I have owned have had grooves that seemed 3/8" wide and about .003 deep. Wide and shallow seems to have been the rule. The rifling in one was straight for the first 20" of the 28" barrels before starting a very slow twist. So slow that when I first looked at it I thought the rifling was straight the whole way. The second had a twist rate of about 1 in 72". The 16 bore combination that I owned had the wide and shallow rifling but was straight the whole way down.