Report on Brady Act and prosecutions
Here, in pdf.
Gist of it: during 2006 there were
8,000,000 plus checks
117,000 denials (125,000 denials, offset by 8,000 reversals on appeal)
77,000 of these were referred to Field Offices for investigation (80% not involving felony records)
273 persons charged, and a whopping
US Attorneys declining to take cases appears a major factor in the drop from 77,000 to 273. Historically, they were skeptical of felon in possession cases, viewing them as street crimes that didn't merit Federal attention. If 80% of cases referred involve non-felons, I'd imagine they were even more skeptical of taking a case where the defendant had a domestic violence misdemeanor conviction, was an illegal alien, had been committed to an institution, etc..