Now, a new study from the non-profit research organization RAND Europe has attempted to provide some more clarity on the trade, estimating that weapons-related listings generate tens of thousands of dollars worth of transactions per month.

The trade "is big enough to generate a lot of noise," Giacomo Persi Paoli, one of the authors of the RAND Europe report, told Motherboard in a phone call.

The research was based on similar methodologies to those previously used to analyze the dark web drug trade, which take scraped data from marketplaces typically accessed through the Tor network. According to the paper, the researchers identified 52 unique vendors selling weapons or similar items such as ammunition, explosives, or components such as silencers, spread across 811 listings and 18 markets (some sites have explicitly banned the sale of weapons).

Typically when someone buys an item from a dark web market, they are prompted to leave a rating; that way, future potential customers can know whether a vendor is trustworthy or not. But for researchers, a piece of feedback is the best indicator of a sale, and each can be used to build up a picture of how much money is being spent on particular items. Listings also often include where a vendor will ship from, and where to, meaning researchers can see which destinations are more popular, or where items are sourced.

"The volume can be considered sufficiently high to be cause for concern."

However, it's worth bearing in mind that spotting a scam in these listings can be difficult: vendors may fabricate positive feedback themselves by simply 'purchasing' their own items and leaving a good review, for example.

From that data, the researchers estimate firearms and related goods generate 136 sales per month, and a monthly revenue of $80,000. Pistols were the most common item for sale, followed by rifles and sub-machine guns, the report reads.

A good chunk of the trades were destined for Europe, while the United States appears to be the main source where guns are sold from, according to the study.

"Europe was a main recipient, both in terms of the number of confirmed transactions ending up in Europe, but also in terms of revenue," Paoli said.

Guns may be small-fry when compared to the overall dark web trade, and especially that of narcotics—the 811 listings the researchers analyzed made up only 0.5 percent of all item listings scraped from the markets—but this space of black market weapons is probably larger than many may think.

"The volume can be considered sufficiently high to be cause for concern for policy makers and law enforcement agencies" the report reads.

Indeed, in its recent Serious Organised Crime Assessment, the UK's National Crime Agency (NCA) pointed to the dark web as a threat in the country's battle against illegal firearms. (The NCA, Home Office, and Metropolitan Police provided testimony and interviews for the report too).

"I really hope this report helps put a little bit of evidence on a lot of the urban legends that are around there," Paoli said.