What you heard will get into effect from April 2018.
What Happened So far:
In recent times, YouTube is strictly constructing its policies to improve the quality of content user want to receive. One in recent time update is, To Enable Monetization, “A Channel requires 10,000+ watch hours and 1,000+ subscribers” instead of “total 10,000 views from all the videos in the channel” [For more: https://support.google.com/youtube/topic/1115889?hl=en&ref_topic=2676320] 
What is going to happen now:
To feel YouTube as a safe place to be with, it’s upgrading the restriction on the Gun-Related video content. Here, the action will be severe like taking action to ban the channel/video completely. 
Detailed Audience:
The motive of selling or promoting intentional for manufactures of firearms and their accessories or guiding someone to build a firearm and firearm accessories of their own will face the consequences from the team of YouTube. It also includes the video’s description that acts as a place to promote the firearm and accessories based website URL.
Where and Who?
YouTube announces it in their very own help forum. https://productforums.google.com/forum/#!topic/youtube/_nbVD4_RvzQ
YouTube Spokesperson told to a marketing firm that, “We routinely make updates and adjustments to our enforcement guidelines across all of our policies. While we’ve long prohibited the sale of firearms, we recently notified creators of updates we will be making around content promoting the sale or manufacture of firearms and their accessories, specifically, items like ammunition, Gatling triggers, and drop-in auto sears. We will begin enforcing these new guidelines next month. We recommend creators take a look at our Help Center and review their own content during that time.”
Why it came into act?
The Expert team of YouTube is working to build the policies for past 4 months. These policies came into act after the mass shooting in Parkland, FL that restarts the debate on gun control over various medium. In recent times, YT has banned some videos that give a demo of “how to convert firearms to fire more quickly?” We can expect more in YouTube Policy Updates in Future, since the audience through internet bigger every day.