Why was it not possible for the bullets from the Quincy's robbery and the murders to match Hinton's mother's gun?

Hinton knew it was impossible that the bullets from the two murders and Quincy's robbery matched his mother's gun. The gun had not been fired in 25 years. Plus Hinton's neighbor had been there when the police got the gun from Hinton's mother. When the detective put a cloth inside the gun and pulled it out, the detective said the gun was full of dust and had not been fired in a long time.