Depends what you're shooting, some surplus ball will go through 3/8" at that range and a lot of modern .223 penetrator might as well. I'd say minimum of 1/2".
There is a High chance of Ricochet, but if you're 600 yards away, I wouldn't worry too much!
I do not shoot ball ammo. I mainly shoot ballistic tip rounds. I would not even attempt a 600 yard shot at 600 with a .223 although it can be made. I was hoping the bullets would splatter into little bitty pieces...
If they are ballistic tips, they will shatter. My cousin brought his 17 HMR ballistic tips and it was making pretty big dents in our quarter inch thick barrel. anything bigger goes right through the barrel.
Yeah it is an old underground barrel. I think it was for oil. Actually now thinking about it it could be 3/16 thick, but still 223 will penetrate both ends of it.
I would suggest getting some 1/4 inch plates, drill holes in
the corners, and bolt together to achieve the needed thickness
based on experience. Three of them would be a good starting point.
As the forward plates are progessively damaged they can be
replaced at a small cost when compared to getting a new
1/2 steel plate. Costs for steel plate rise dramatically with
thickness and 1/4 inch is generally readily available/affordable in most locales.