I asked for a couple reasons, DJ has suggested using a .490 round ball for the slug, cheap and fairly easy to come by. Second, your thread reminded me to do something, whihc was take my bore gages and find my bore diameter, .451. This is good. One side of the gage is larger than the other so it doesn't give a reading I trust of the groove diameter, but I got a repeatable .457, but it could be .460 for all I know.
I've ordered that 38-55 barrel, and from everything I have read, this barrel has some weird dimensioning going on and can be quite a challange to get to shoot well. So I'm about to move up a level or two in my cast bullet shooting. Up till now, if the bullet could be made to shoot well at any reasonable velocity I was happy, but I want to get a 2000 fps load or better out of the 38-55 so I'm gonna have to up the ante in my casting efforts and the 45-70 seems a good place to start at present.
I made up some 20:1 last weekend, and found it casting undersize, went back and checked out bullets from past sessions and sure enough, undersize. On another board, DJ said that mold release made his molds drop undersize by .001, I was seeing .002-.003. I cleaned out my 340 mold and sure enough, it started casting .4565-.4575, can't get any better than that. Went out and shot three, and fired a 2" group where 3 was the best previous with that load, my eyes and iron sights. Gonna shoot some more tomorrow and see how I can group 'em. The last I was shooting that bullet was with a scope (that wasn't busted) and I was getting 2" 5 shot groups with 3 in clover leaf to 3/4" clusters, but no rhyme or reason to the order that made the cluster
I've rambled enough Ian, and can't seem to get where I wanted to go, sorry to take up so much of your post,
John