I bought a Lyman Foster mould about 30 years ago and hate it!
It is and always has been a pain to cast with mostly because the slugs stick to the core pin. I could taper it more and polish it but just haven't bothered.
As said above, the slugs cast small and seemingly range from 0.685" to 0.705". Mine casts at 0.705" so too big for shotcup and too small for the bore. I found that when cast soft and shot into deep snow recovered slugs showed uneven bump up in that the noses were all cocked differently and the skirts tended to have uneven trailing edge so accuracy was poor. My groups ran about 8' to 12" at 50 yards.
I did manage to get some decent accuracy by using old brown paper shopping bag paper patches. That tightened up groups considerably but I still got fliers... likely patches not liking crimp opening or maybe splitting during the transition from hull to forcing cone as the slug bumped up... or patches not coming off properly.
And yes, if cast from soft lead they will bump up.
As tomme boy said, Lyman used to sell swaging dies to put rifling on these slugs and I thought that maybe once swaged that they were closer to bore diameter but someone here found a set of those dies and the finished slug was well under bore diameter so I have no idea why Lyman would make a mould so undersize... especially if the slug bump up to bore diameter anyway!
I thought about machining driving bands on mine to bring it up to bore diameter but decided not to bother. I just made my own bore diameter mould. Casts better and much more accurate. Buckshot (on this site) machined a Lyman Foster slug mould to add 4 driving bands many years ago and posted pics.
SluggerDoug wrote up his "knurling" process to expand the Lyman Foster slug to bore diameter:
https://www.shotgunworld.com/bbs/viewtopic.php?t=132184
and you can see Buckshot's machining job here:
http://castboolits.gunloads.com/show...n-foster-slugs
post #10.
I have been told that if the slug is cast from soft lead and set onto a hard wad column that it will bump up better and accuracy can be quite good. I was not successful at all. I gave up on that one.
Sorry!
Longbow