YOu probably need cast bullets at least .430-.431 in diameter . Did you drive the slug all the way through the barrel and then take another measurement ? That will tell you a lot , trust me !
I still suspicion you have a tight spot where the barrel threads onto the frame . It will feel like you hit a brick wall with the slug when you get to that junction point and if there is a tight spot there .
Don't get too caught up about oversize throats . There have been experiments where they took the barrel off completely , fired cast bullets into a bullet trap . they measured the base before and after being fired. It had been " bumped " up in size by the pressure of the fired round .
Remember , it's better to have slightly oversize throats and undersize . Let's take the time to drive a lead slug all the way down the barrel before buying more cast bullets of different sizes . I feel that you are just throwing away good money otherwise . I think you will find the trouble . I suggest spending $15 or so and buy veral smiths booklet , " Jacketed Performance with Cast Bullets " . Or , as I mentioned above try some plated bullets from someone like Xtreme , Berry's , Ranier etc. That could be an easy fix for you . Shooting cast is not difficult , once you learn how to check for trouble area's and how to remedy them . It's just a journey in education and like I just mentioned , Verals Booklet explains it all and how to fix . I'm simply sharing many years of experience in shooting cast from revolvers . So Daimler 1989 , there you have it , several possible suggestions , I hope I have helped you .