Oswald never fired a rifle that day as tests showed
What tests are you referring to? The paraffin tests? Are you aware that the FBI test fired CE139 to see if it would deposit nitrate residue on the cheeks on those who fired it?
and they didn't find his print on the rifle before the chain of evidence was broken.
Wrong. Oswald's palm print was lifted off of the rifle on Nov 22, 1963 by Carl Day.