It actually isn't so bad. Cecil is probably about right though in his assessment of what will shoot correctly in one. Have a "GOOD" barrel is even more important than the twist rate and many of the original 1:20 .45 caliber rifles (especially the Encores) had barrels that would have been terrible no matter the twist rate.

Dave, the White .451 rifles use 1:20 twist and many are absolute tack drivers.