Well being in illegal possession could taint how the DA treats you. Some DA like Mike Nifong might feel the need to make some example of you for political purposes and throw a murder charge. If they felt like it they certainly could charge you with the illegal gun charge.
Here is an odd source but illustrates the latter scenario, well not exactly but as close as I could find on short notice.
Prosecutor says Proof shooter within law
Shooting was in defense of another according to prosecutor The man who police say shot and killed rapper Proof in a nightclub shootout acted lawfully in defense of another man, the county prosecutor said Tuesday. But Mario Etheridge still will have to face weapons charges in the April 11 incident in which Etheridge's 35-year-old cousin Keith Bender also was killed.
Etheridge's lawyer has said his client fired a gun while trying to defend Bender during the April 11 fight at the CCC, an after-hours club on Detroit's Eight Mile Road... [The prosecutor] said Etheridge shot twice at the ceiling to stop Proof -- who was a member of D12 and Eminem's best friend -- before aiming the gun at him...
...A message seeking comment was left Tuesday for Etheridge's lawyer, Randall Upshaw. Upshaw has maintained that the 32-year-old Proof — who died with a blood alcohol level of 0.32 percent (four times the level considered drunk under Michigan law) — became belligerent during a game of pool. The man he argued with stopped to speak to him on his way out, Upshaw said, and Proof then argued with Bender and later returned with a gun, pistol-whipped Bender and shot him in the face