The .357 barrels I have used are 0.350" land and 0.357" groove, TJ's use a 26" twist.... In .457 cal they are 0.450" land and 0.457" groove, TJ's use a 24" twist, which should be fine for pretty much anything airgun usable.... They are both 6 groove.... Some airgun barrels are only .452 cal, which is typical for pistol barrels and it might be easier to find short, light bullets in that size, and of course you could use a much longer twist rate.... but that would not be my choice.... If you want to make sure the longest and heaviest slugs will work, I would do a 24" twist in both calibers.... I have some ideas about the rifling profile if you want to email me....Bob