I bought a .223 Bull Barrel a while back and did the same thing you are doing. I went to Wally World and bought 2 boxes of the UMC stuff to break it in.
Truth be told, the stuff shoots great out of my gun.
I have shot some very tight groups with the UMC ammo. Not everyone has the same luck with it but for me it shoots good.
I also shoot some dirt cheap Barnaul ammo. This ammo shoots good but not as well as the UMC. I don't know why but my gun likes the UMC better.
I haven't tried any expensive ammo because the cheap stuff is shooting good for me.
I have since bought another Rossi. A 7.62x39. I usually buy Handi Rifles (I have 10 of them) but I could not find a 7.62x39 Handi.
Crazy thing about this one is, it loves the Barnaul ammo (123 gr. Hollow Points). I shoot tight groups with this ammo and it is dirt cheap too!!
Now these are the only 2 Rossi's I own because I really love my Handi's but I gotta say, these things are really good guns and they are cheap (I paid WELL under $200 out the door for each of them).
Now I know there is alot of H&R vs. Rossi talk but I'll be honest, my Rossi's shoot every bit as good as my Handi's (The Rossi's do look a little funky though!!)
What are you planning on using for a scope?? I have 3-9x40 Bushnell's on both of mine. I like the Bushnell's for their brightness.
Good luck with your .223 and happy shooting. Give us a range report when you can.
Spanky