Dabbler, I also have a M12 with Hastings, mine has screw-in chokes. Over the last three years I have shot a couple of cases of Kent Faststeel 2-3/4" # 1's throught the modified choke, no problems ( and lots of dead geese!) I don't know if it's actually rated for steel, but in my opinion with good shells, ( no bigger than BB) and nothing tighter than modified choke you shold be OK. You minght try contacting Hastings though for a definitive answer. Most problems with early steel loads were due to poor wads (barrel scratches) and very large shot through full choke (ring bulges)!
Tungsten Matrix is safe for any modern gun, as it is only as hard as good lead shot. I use it for late season shooting for the extra reach.