I'm thinking your chambers may already be honed too large if you are getting blowback. I don't use magtech shells, but the various types I use (thin Winchesters, medium thickness Alcans, thick RMC) never cause blowback in three shotguns that have had minimal to no honing (TTN, Liberty II, Baikal). Never have to clean the chambers during a match. This is with a medium load of 60gr Goex. Plastic hulls always fly out, the brass always do except for a few that are a little out of round so they stick a bit going in as well.
Possibly something like this would size your shells better:
http://davidscottharper.com/shoot/SizingTool.JPG That is a tool I got from Nate Kiowa Jones. The steel plate is 1/4" thick or so, and the two holes are .790" and .795", with the holes chamfered on one size to help get the shell started. Rarely need to use the .790" size. I lay the tool over the open jaws of a vice, pound the lubed shell into it with a rubber mallet, then turn it over and put a dowell inside and pound it back out. I only need to do this occasionally on some problem hulls.
Good luck.