A picture would help with what you are describing but my bet is that is just pressure ring expansion.They all do it in the 45-70 unless it is something like a 400 grain factory load.The more it expands/swells the more pressure..Garrets semi mild loads also expand/swell at under 35,000 CUP and Buffalo Bores expand/swell more.
When using the same brass as a known load,I have measured the expansion for comparison...Nothing to worry about if your within the data.
Which 45-70 brass are you using?
Jayco
