Late Thursday, REI issued a statement noting that while it does not sell guns, it expects companies that do to help prevent mass shootings.
MEC, founded in 1971, is a consumer co-operative that sells health, fitness and outdoor living products to members.
"We had a lot of customers after the shootings in Florida come to us and say 'What are you doing listing a product made by an ammunition company?'" Running Room founder John Stanton said in an interview. The petition had collected almost 50,000 signatures by Thursday morning.
"Thousands of MEC members have contacted us to express their concerns and to ask that we stop selling products made by these brands", said MEC Chief Executive Officer David Labistour in an open letter published Thursday.
Representatives from Vista did not immediately respond to HuffPost's requests for comment.
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MEC does not sell guns, but it faced an online petition calling for it to stop selling brands from Vista, which also develops and makes firearms similar to the type of rifle used in a recent USA school shooting.
WhileREI doesn't sell guns, the retailer carries products from a number of companies owned by Vista, such asCamelbak and Bell.
Retailers aren't the only ones taking action. "As a result, we have chose to place a hold on future orders of products that Vista sells through REI while we assess how Vista proceeds".
"REI does not sell guns", said the company in a March 1 statement. Another outdoor retailer, Mountain Equipment Co-Op, cut ties with Vista Thursday as well. In a regulatory filing, Vista said it owns three gun-related brands with top market share in their categories: Federal Premium in ammunition, Blackhawk in gun holsters, and Hoppe in gun cleaning accessories.
A number of USA stores have recently changed their gun policies by raising the age bar for buyers to over 21 years and taking the assault rifle AR-15 - which was used to kill 17 people in the parkland Florida shooting last month - off their shelves. On Wednesday, Dick's Sporting Goods announced that it will stop selling assault-style rifles in its 35 field and stream stores (Dick's had stopped carrying that style of gun in its other stores in 2012, after the Sandy Hook shooting).
Retailers like Kroger Co and Walmart Inc have said they will stop selling firearms and ammunition to buyers under the age of 21, while transportation companies like Delta Air Lines Inc and Hertz Global Holdings Inc have severed ties with the National Rifle Association gun rights group. "We encourage Vista to do just that", the company concluded. Camelbak and Giro have also sought to put some daylight between itself and Vista.