Modify Brass with a Mini-Chop-Saw

Drill Master - 2 in. Mini Bench Top Cut-Off Saw:

Works well for cutting brass for caliber conversions ...

https://www.harborfreight.com/2-in-mini-bench-top-cut-off-saw-62136.html
Of course a Dremel-tool with a custom jig can do this also.

Or even a table or radial arm saw using a jig to hold several shell in position.