I have done a lot of fire forming of bullets, and the most important part is not fireforming.
Annealing is far more important than the fireforming how to. Learn to destress your brass and do so before every step of your fireforming, even new brass is better annealed before fire forming.
That done, then shooting it.
I have a 210 liter oil drum plastic rubbish bin in my garage which use when absolutely full as a fireforming silencer. The full of garage and household rubbish drum has a hole barrel size two thirds down the side and a thick old blanked foldeda few x over the top with a weighted lid over that. When I fire form I just place barrel in drum and do the deed, its no louder than dropping a phone book flat on a tiled floor or hitting the last hit on a nail when you hit the wood as well. I phone closest neighbour, tell her I will be doing some wood work, please let me know if I bother you. She claims not to have ever heard me. My wife sleeps about 4 rooms away when I work, no hassle.
I have used Mp - MS to achieve desired results, I also have a bottle of spilt propellant, droppings and leak out autoloader and Lee pro press. These droppings are everything hand gun, rifle and shotgun propellant.
I usually use 12 - 20 gr of mix for fireforming and I have topped charge with almost everything you can imagine, cotten, kapok, toilet paper. I have use Porridge, pronutro and birdseed, sawdust and shavings to a small load of birdshot. I have stuffed a case full of cotton wool and topped that of with soap forced into neck, I have poured wax in and used 2 mm of polifiller. I have used silicon sealer and even a small amount of Bostic contact dribbled on top of paper at mouth of the neck.
They all work, every way of making a bang works, sometimes just a tweak is needed to raise the resistance- pressure.
....................But please, anneal thos cases.