Shopify Inc. (SHOP.TO), the Canadian provider of online stores for more than 600,000 businesses, banned the sale of some semi-automatic firearms and 3D-printed guns.

Semi-automatic guns that can take magazines that hold more than 10 bullets, attachments that make guns fire faster -- like bump stocks -- and silencers or flash suppressors are all considered “restricted items,” according to the company’s rules, which were updated Monday night. The popular and controversial AR-15, as well as other “assault-type weapons,” and most pistols would fall into this category.

The move is a new area where Shopify, whose stock has rocketed more than 700 per cent since its 2015 initial public offering, is grappling with the responsibility that comes with being the internet’s dominant platform for small merchants looking to sell online. By banning weapons that are otherwise legal to buy, the company is pushing into one of the biggest political and cultural fights in the U.S.

“Solely deferring to the law, in this age of political gridlock, is too idealistic and functionally unworkable on the fast-moving internet,” Chief Executive Officer Tobi Lutke said in a blog post. “We have found ourselves in a position of having to make our own decisions on some of these issues. And along the way we had to accept that neutrality is not a possibility.” Inc. has long banned merchants from selling firearms on its site. That’s made Shopify one of the go-to platforms for people selling guns and gun parts, said Dustin Pitcher, who runs and uses Shopify.

“This is the market where Shopify doesn’t have to compete with Amazon,” Pitcher said.

Shopify said the number of people affected by the change is “small.” Pitcher isn’t so sure. He has annual revenue of about USUS$2 million and says there’s a whole ecosystem of similar stores that sell guns and gun parts. Because the issue hits home for many conservative Americans, Pitcher said it’s possible other brands that don’t sell guns but cater to conservatives might also abandon Shopify.

“The overall size of the market I’m in is much bigger than people think it is,” he said.

Last year, activists pressured the company to stop serving conservative news site Breitbart’s online store. At the time, Lutke said commerce was part of free speech and Shopify wouldn’t kick Breitbart off. Products that specifically promote violence against people based on their ethnicity, religion or gender are already banned.

The company’s U.S.-traded shares fell 3.8 per cent to USUS$138.92 at the close in New York.