You  could do that but I just load them with regular bullets/loads and fire. Mostly reject bullets used to zero or plink.  Assume since you are breech seating not many cases to form.   

Boats

I've been forming .458win mags from 7mm Rem mags this last week.  Have to open them up to .375 or .400 with expanders before blowing them out, but it's working reasonably well annealing several times in the process.

I drop in 7 gr of a WW shotgun powder (a friend uses 5 gr of Unique) and fill with grits (my friend uses rice).  7 gr is a stiff charge, you can do it with less.  Start small and work up.  Results are good with no hint of where the shoulder was (other than color).

Grits are messy.  using a lead bullet works well if it doesn't lead your barrel.  I used 30 cal bullets in blowing out .30-30's to .375 Winchester.  Took a long time scraping all the lead out.