To build on this - casings and bullets are more likely to be useful because the marks left on them by the lands/grooves (for the bullet) or the chamber, extractor, and bolt face (for the casing) are likely to be consistent with the criminal’s firearm. However, this assumes that the criminal is caught first, and the gun they used was both recovered, and can be linked to them. (either circumstantially or by forensic evidence).

I suspect that, if you replaced the barrel and extractor immediately after committing a murder, it would be very difficult to link the firearm to the murder, particularly if it was an exceptionally common firearm. Although you’d want to buy the parts with cash, so that the purchase couldn’t be linked to you…