If you know somebody that makes bullets get a few cores from them. I put a core between Deltronic pins and use a brass rod to tap on one pin to upset the core. I realize that few people have access to Deltronic pins and such but you could probably pull a couple of rimfire bullets and use one of those. You could get some 3/16" brazing rods and use them to upset the slug.
If that doesn't work I'm fresh out of suggestions. I can send you a few cores if you'll give me your mailing address. No charge.