Originally Posted by YubaRiver
Are you talking about gun powder? Where charges can be measured to the 1/10 of a grain? (1/70,000 of a pound). Messing with charges by weight or volume could be deadly. And early experiments with the smokeless powder Herco showed problems with spontaneous combustion with taggants. And you want plastic going down the barrel of your rifle? Wonder what kind of solvent you would need to get that out?
I don't know what kind of chemicals are proposed for the tagants, and I'm not an expert in that area, but I can think of some things that would usable and would be a fraction of a milligram and wouldn't cause any barrel fouling. I would do something with isotopes, like pick a few elements that have, say, half a dozen isotopic variants each, and then use the ratios to do a tagging. It would not be easy to analyze this stuff but it would be possible and it would have no impact on shooters. A few micrograms of rare elements per loading would be all that's necessary, and that would have absolutely no impact on shooters, and operating cost to produce it would be very small. You could encode thousands of different batch IDs this way, so it would let investigators narrow things down, but not to the level of an individual purchaser or box of cartridges etc. Often investigators get a big help from narrowing things down, so this would be valuable.
One problem with this idea is it could be messed up completely by mixing a few different batches together when making a bomb, but most AQ terrorists lately have been buffoons.