Of course this method will be perfectly well used to catch those WHO BUY ILLEGAL GUNS AND AMMUNITION. Because obviously, all the criminals will put the serial number on their guns and their bullets.
Do you think criminals make their own guns?
Remember, these are people who can't even hold down a job, in most cases. Initiative and industriousness do not characterise most drug addicts, gang members etc.
None of the bad guys are going to change their behavior. Few, if any, of them go to the local gun shop and plop down 650 bucks for a Colt and a box of laser etched ammo. I'm suprised you don't see the problem with this idea. Gun laws only affect the legal gun owners.
Of course most of them won't buy a laser etched gun from a gun shop. Figures show less than 20% of criminals in the US get their guns that way.
What most of them do is
steal the gun the law abiding citizen has bought, or buy it from someone else who stole it.
Whatever the law abiding buy, that's the type of gun criminals end up with, because that's where the criminals get their guns.
As to ammo, criminals again either steal it, or usually just buy it.
Make a law that requires serial numbers and it will feed through to the criminal guns in time.