Didn't the FBI (or some other law enforcement agency) re-test Burke's guns in the last 10 years or so?

I thought I read somewhere that they decided to check them again using more modern ballistics / forensic tools and techniques, specifically to see if they could be matched to some other cases in Detroit. My memory on this is hazy, and it may not have been Detroit, but it was a city other than Chicago.

And, jonnynonos is correct, it was likely Burke, Winkler, Bolton etc. I think the evidence against them is convincing.