Makes me wonder if jamming the lands isn't enough to keep the case from moving. Seems like it didn't seal, leading to the change on the shoulder. Maybe a false shoulder would be a better option.
When I make 6.5 Sherman brass, due to excessive shoulder movement, I need a false shoulder to ensure correct headspace when forming. To create a false shoulder, you size the neck down in increments just enough to allow you to chamber with a "crush fit". I usually use 280 Rem brass, but have used 270 Win as well. In my experience, the 270 neck is too close to the 6.5 to get a great false shoulder, so I prefer starting with 280 Rem.
In a similar fashion, you could take 7 SAUM, expand it to 30 cal, then I would slowly size down the neck until you can chamber with a crush fit. This *might* yield better results.