is there any harm in shooting steel case ammo out of my AR (just for plinking I wouldn't do anything meaningful with the stuff)? also what bullet weights in your opinions are most accurate out of a 1in9 twist 16" barrel(.223)?

I will bet you are going to get a lot of different opinions on this one!
The soft steel case will not harm any modern firearm.
Most seem to shoot the heavier bullets better
69 vs. 55

You are no doubt talking about Wolf ammo. The case is lacquered to keep it from rusting. Some say the lacquer melts off and fouls the chamber after sustained, high-rate firing. My experience is that the stuff is dirty-- requires a really good cleaning after you use it. Some claim it precipitates stoppages because its dirtiness clogs the gas port. It also has a funny smell-- unlike any other powder I have ever smelled. Best part of Wolf is that you can't reload it-- you just let it lie...

I bought a S&W M4 5.45X39 AR-15 last year and enough steel cased Bulgarian ammo to wear it out. The rifle will never see a brass case in it's life.

I bought it as a replacement for an AK-47 I owned with the sole intention of spraying ammo with it and having a good time. The amazing part of the whole deal is I can shoot MOA groups with the ammo!

I personally wouldn't run steel cased ammo in a target AR but for a shooter, yes, in a heart beat.

Terry