fmj should be just fine and steel cases ar,nt that bad its still softer that your gun metal, it just cheap ammo and clean the gun afterwards.....
My only concern with steel ammo, especially the Russian brands do have a polymer coating sprayed on the cases which when fired through a hot gun will leave deposits in the chamber and mechanism. I've seen AR's lock up and fail to fire after this stuff was shot through it with regularity. They say they have changed the polymer formula and I haven't heard one way or another if it still is problematic. The cases are also harder, including the primers which have been known to fracture firing pins and break ejector claws right off. Outside of that, I guess it's okay to shoot... LOL!
Re: TulAmmo .40 S&W Steel Case Recently, I read a thorough study on brass vs steel in one of my gun mags. After many other, of course. The study ran various inexpensive, range-type ammo, and I believe they were using 10k per AR. ARs were identical & all shot the same drills at the same time. The metal casings had more failures: FTF, FTE, etc. One of the carbines using steel case broke-down at just over 6k rounds. The other AR using steel case almost made it to 8k rounds before the barrel went (most steel case ammo uses steel pin bullets). There were some ejector issues with the steel case as well. The identical ARs using brass casings, all made it to 10k rounds with little difficulty. One of the most common problems with the cheap ammo was inconsistent charges, which caused FTF & FTE. The cheapest I have is a few cases of Silver Bear .223 62 gr, HP. They have metal-stem bullets so no indoor range will let you use them. I tried, but they brought-out the 'magnet'.... nuts!....looked like lead HPs, but the magnet picked 'em right up. Well, I pushed ALL my steel case to the back, and will only use the "steel case" if/when I run out of e/thing else and can't get any more. There is prob no cost saving when using the 'Cheap-Junk-Ammo' as it does do real damage to decent firearms, which can be quite expensive to repair. The junk ammo can wreck an extractor to trash a barrel, over time... so there goes an savings. Also, MY ARs are NOT for hunting groundhogs, they are for fighting against Tyrants who would violate our U.S. Constitution... enemies of the U.S., foreign and domestic.