YouTube has banned videos that show people how to manufacture or modify guns and their accessories.
It had already banned videos linked to the sale of guns and accessories.
Many firearms enthusiasts noticed that some of their videos had been removed from the video-sharing website and some had their channels suspended.
Prominent gun video-bloggers said the move was an erosion of US citizens' rights, and some said they would move their content to PornHub instead.
YouTube's policies now prohibit videos that:
- show how to make a firearm, ammunition, high-capacity magazine or homemade silencers
- are designed to sell guns or specific accessories including high-capacity magazines and tools that convert a firearm to automatic fire
- show how to convert a firearm to automatic or simulated-automatic fire
- show how to install such accessories or modifications
The changes will fully take effect on 20 April.
The decision was met with anger from some videomakers who modify guns and show off their creations as a hobby.
Karl Kasarda and Ian McCollum, who run the gun review site InRangeTV, said they had started posting their videos on Facebook and pornography site PornHub.
"We will not be seeking any monetisation from PornHub... we are merely looking for a safe harbour for our content and for our viewers," the pair said in a statement.
Firearms manufacturer Spike's Tactical said the change reflected attempts to "slowly chip away at our freedoms and erode our rights".
Videomaker Joerg Sprave said he appreciated YouTube was "now defining their guidelines" more clearly.
But he said the change had been introduced without a transitional period.
"Many gun channels must now be afraid," he told news site Motherboard.
"They should at least get some time to clean up their videos so the new rules are kept."
Unsuitable for children
On Tuesday, YouTube was criticised after the Sun newspaper found step-by-step instructions on how to build an air rifle on YouTube Kids, the company's app for children.
Despite being designed for children, its content is curated by algorithms. Inappropriate videos have repeatedly slipped through the net.
In February, the BBC's Newsround programme found instructions on how to sharpen knives on YouTube Kids.
At the time, YouTube said it had a variety of processes in place to try to prevent inappropriate material appearing on its platforms.