A 300 Blackout magazine should just be a .223/5.56 magazine and maybe it would feed the first round if you're really lucky. Otherwise no it will not work well or function normally. The cartridge dimensions are very significantly different.

Aaron, Not really. What 7.62 x 39 gun will your friends 300 BO P-mags fit in? You could load about 5 - 7.62x39 rounds in a 300 BO mag and maybe have it work if mag fit in firearm. The rounds would be canted up or down. The 7.62 round have a totally different shape and size. That is why the 7.62 x 39 mags have such a curve.

sorry it will not work

Aaron, no Sir. AK and AR 7.62x39 magazines have a specific curve for a reason. Cheers and stay safe!