3. Just as a general rule, I would NEVER rely on brass that has been "resized" before I procured it to be actually resized. Never. Not once. You are taking a gamble with that.
I agree with this. If you bought this brass with the understanding that the brass was sized, primed it, put powder in it, and now found the bullets are loose, that ought to tell you the neck was not properly sized.
Maybe some of it was, which maybe why some bullets have a nice tight fit.
The next time you buy once fired brass, you size, trim all of the cases, in your dies, regardless of what the seller said he did with it.
And use small base dies on 223 rounds to be used in gas guns. Many of those once fired military rounds were fired in machine guns which have chambers so large, they use sausages as headspace gages.