Sure, they are the easiest to get compromised, not all do though. While undercover officers would probably be best for busts of that nature, in the OP above I was referring to cases where they use informants, and in at least some cases the police might not want to use one of their own as the suspected arms dealer may already be aware of all of the undercover narcs on the local police force (black market traders have some complex ways in countering police efforts) or have their own spys who closely monitor local police activity.

Some years back I had worked with a few guys who were on work release, and on the side even while still incarcerated, they dealt in drugs and weapons, and they had ways of being able to determine if potential customers were undercover police, and even pointed out some of them to me (not that I cared, as I wasn't involved in criminal crap like that). I think the police know that, which is why a good number of them rely on informants to help out, and when going the informant route it sounds logical they could run into the problem mentioned in the OP.

Also, in case you wonder what brought this post on, a few nights ago in class one of the police detectives giving the presentation went into detail regarding drug and illegal gun busts, and why the latter of those two is usually more difficult for them to pull off, and based on what he said I've no reason to believe he was lying or exaggerating.

@SpikeTalon I would say the main reason for the lower number is the smaller market for illegal firearms. There are a number of legal firearms and legal firearms dealers and legal firearms purchasers. There is a huge difference in the way the law is applied between drugs and guns. I can possess a semi auto, a conversion kit and ammunition in any quantity without violating the law. If I buy the precursors to drugs whether I combine them or not, I can be arrested. I can buy a license to manufacture fully automatic weapons, I cannot do that with drugs. It wasn't a true apples to apples comparison. The original premise may be correct, but it is not a fair comparison.

@glennlab You're right about the comparison part, and precisely because those two could not exactly be compared against each other is maybe a reason why the data for the two regarding busts should be considered individual of one another. That's pretty much the reason for this post, certain factors to consider, such as what you pointed out with how the laws are applied.

@SpikeTalon It's like comparing the number of kills for duck hunters versus moose hunters.

Yes, and that's why they have SWAT units for such scenarios.