Load your favorite .300BLK in these mags and I doubt there will be issues.
PS: I have been asked where can people buy those magazines. I do not know but there is a rumor that certain Florida Gun Blogger does the hydrographic kitty thing in the comfort of his Abode. 🙂
Here is some of his previous work:
OK, before he shoots me, he really does not do kitty magazines… but I could not resist semi-starting the rumor.