See Stan Waters' PET LOADS. To "fireform" .30-30 to .38-55, do it like this;
1. pick up lots of abandoned .30-30 cases at your range and clean em up. Sizing is not required if they fit your chamber. I'd sort them to get a uniform lot of cases.
2. prime them
3. 15 grains of BULLSEYE
4. Cram as much paper towel into the case, on top of the powder, as you can
5. Go to your range, so the neighbours won't get upset
6. Load & fire into the air, so the crumpled paper don't hurt anythin'
If you just use an expanding plug, the end result is shorter than you'd like. Works for .32 Spl & .32 Wcf as well.
I suggested .375 Win cases on another board. Got spanked, cause there is a danger of someone mixing them up.
1. .375 chambers are shorter, and real .38-55 cases won't fit
2. .375 Win runs at a vastly higher pressure range, and the rounds look alike. If a real .375 Win gets in a .38-55; - LOOK OUT!