Sorry just read your post here. First thanks for reading mine.
It looks like most of your ?'s have been answered. I did pattern my 410/45lc without the choke and it was very thin. I think 15 to 20 feet might be max for a rat or snake.
I have read a very good article in "Handloader" about making snake rounds with 45lc brass. It is true the #12's worked best mostly because there was more. Starline makes the standard size 410 all brass hulls but nothing in 3". Most I have read states that the 444marlin brass is the easiest to use for 410 shells. It is thicker and will last a whole lot longer at 410 pressure levels. A good source of info on loading brass shotshells ishttp://www.endtimesreport.com/410reloading.html
Mike Venturino, think I spelled wrong, used copper gas checks between the shot and powder and on top of the shot with a slight roll crimp for the 45lc.
I haven't tried it yet, it has only been a thought, but I wondered about cutting the end off a piece of brass, filling with #7 1/2" and using LEE liquid alox to "bind them together. Let them set and carefully pushing them as a unit into a charged brass shell and putting a gas check over them with a slight roll crimp. Would come apart pretty easy, but might hold a little tighter that way. Again it is just a thought that I have never tried. Yould could even mix some up in a container and scope them into a charged case,again make sure a wad or something is between them and powder and let it set a couple days before seating another gas check and crimp.