I plan on building a .300BLK AR upper and was curious if there were any stamped .300BLK 10rd pistol mags in Canada for a legal 10rd limit, I could not find anything after some searching. I don't think the XCR pistol with a .300BLK upper, if there ever was one imported into Canada, had a 10RD pistol mag like for the 5.56. I'm pretty sure it would have just used the regular 5.56 mag. There may already be stamped .300BLK pistol mags already available for purchase in Canada, but I cannot find them.
I have noticed a recent trend moving towards manufacturers designing mags specifically for .300BLK to increase feeding reliability; must be increasing in popularity or manufacturer's wouldn't be investing funds to create these which is a good sign that .300BLK could become more inexpensive down the road. I wonder if there are any U.S. based manufacturers that are stamping there .300BLK mags for pistol use like Robinson Arms has done with their 5.56 mags for the XCR. Ruger just came out with their new .300BLK AR platform pistol; the pistol is using magazines marked .300BLK, maybe its stamped for pistol use?
I think I already know the answer, but what are the legalities of inserting .300BLk cartridges into a stamped 10rd 5.56 AR platform pistol mag and using it in a 5.56 or multi caliber stamped lower receiver with a .300BLK upper?
Nothing you've said makes any sense. Better do some more research.
300BLK and 223 use the same magazines, there is no difference at all even though you can get them with a 300BLK stamp on them.
There is no trend recently, this has been going on for years.
I hope you already know the answer to your last statement because I can't understand where you're getting these ideas, who's been filling your head with fragments of information that make you think the magazine has to be marked the same as the cartridge you are using.
Short answer, buy yourself some 223 pistol magazines and use them for your 300 Blackout. All of the magazines you use in your 300BLK will be 223 magazines regardless of what is stamped on the side.
Why do you want a 300 Blackout? Will this be a restricted firearm or non restricted? Not trying to say there is something wrong with the cartridge, I have two rifles chambered in 300BLK and have owned others. What I'm getting at is that if this will be a restricted rifle for use at the range only there is really no reason to be shooting 300BLK over 223. 223 is a better cartridge for target shooting and general plinking due to the cost per round and the flatter trajectory. 300BLK shines away from the range and in the hands of a handloader where you can take advantage of the versatility of the cartridge. The factory offerings for ammo are not as plentiful and cost more than 223.
That's good news. From my research, I think .300BLK cartridge feeding issues in magazines are caused primarily with heavier weight bullets. Can you confirm if reliability is significantly improved with 110gr and 125 grainers? Are you aware of any specific .300BLK mags for pistol use that have a legal round limit of 10 up here in Canada?
I've run everything from 110 grain through 208 grain projectiles through 4 different semi auto 300BLK rifles over the last 5 years and have never had any feeding issues with Pmags or metal STANAG magazines. As far as I'm concerned any specialized 300BLK magazine is a marketing gimmick to make you pay an extra $10 for a stamp that says 300BLK instead of 223.
Just use 223 mags and worry about feeding issues later if you have them, not worth worrying about before you've even built the rifle or fired a single shot.
Legally you can use any magazine that fits in the rifle regardless of any markings or stamps on it as long as you don't have to modify it to make it function.