BudsGunShop.com - Work with a 300 blackout

Yes it will work. 300 black is a necked down 556 case loaded with a 30 cal bullet, they can use the same magazines, upper, and lower the only thing different is the barrel.
I have seen some people say that 556 magazines don' work relaibly with their 300 black out but these are the min,..ority, the majority of people have zero issues using 556/223 magazines especially magpul.
I would only order one just to be sure but it will almost certainly work for ya.---------
One thing I just noticed is thats the 557 magazine, this one is slightly different then the standard pmags still will run in most AR's but is designed to run in non colt style AR's
------ magpuls website says this
"Modified internal and external geometry also permits operation with a wide range of non Colt-spec platforms such as the HK® 416 and MR556A1, M27 IAR, British SA-80, FN® SCAR™ MK 16/16S, and others. A redesigned bolt catch notch in the rear of the magazine provides increased bolt catch clearance, while an over-travel stop"
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I'd get the standard pmags as these will fit standard AR's better (including 300 blackout)
40rd here
budsgunshop.com/product_info.php/products_id/1130 />30rd here
budsgunshop.com/product_info.php/products_id/114290 />

Matt, it is highly likely to work but I would get the Magpul .300 BlackOut specific magazine here (out of stock at Buds but can be found at the 2nd link) : budsgunshop.com/product_info.php/products_id/114333/magpul+pmag+m3+300blk+30rd+blk ------------------ https://gun.deals/search/apachesolr_search/840815117223

Magpul does make a 300 black out specific pmag
"The PMAG 30 AR 300 B's distinct rib design and smoother upper-half texture provides the shooter a distinct visual and tactile difference from any other PMAG, Magpul Industries, Magazine, M3, 300 Blackout, 30Rd, Fits AR Rifles, Optimized Internal Geometry For 300BLK Bullets, Distinct Exterior Rib Design To Mitigate Crossloading, Black Finish"
These are out stock pretty much everywhere though.
Mr guns and gear has a good video on the subject too.
https://youtu.be/5UwILeYXZlI