Well they can clean the gun for fingerprints by just wiping it off real good but even if they clean out the barrel the testing will still show a match in the test firing. Also, they have chemicals that will reveal blood that has been wiped off. You would probably have to disassemble a firearm to clean it well enough with some strong chemicals to keep it from showing blood that had been splattered on it. The barrel of a firearms is "rifled" meaning it has grooves in it that will make the bullet spin when fired. I'm not an expert on it but it is my understanding that the marks on a fired round is much like a fingerprint. Every firearm creates unique marks on the bullets when it goes through the barrel. Also, unique marks are created on the cases from the powder and even from the way the bullet is ejected from the chamber. This is very solid evidence in a court and has been around for many years. It would be real easy to tell that a firearm had been cleaned. Even an novice could look down the barrel of a firearm (unloaded of course!) with a light and tell if the firearm was clean or dirty. But this will have no effect on the test! It would be like washing your hands before having your fingerprints taken!