It was the mag that came with the rifle so I am not sure what it is called. I very easily could have overloaded the mag, my allergies were bothering me and I lost count so I just filled it until it stopped. I will check it out the next time I go fire it. Thanks
Pmags will not take 31, not from what I have ever seen. What you WILL find is, with most magazines, they need time to seat the spring and compress to allow proper function with a full load out. Time and again, I have seen new 30's jamming up with the first 2-4 rounds when loaded full. But, if you stop the load at 28 or 29, they seem to seat the spring well, and function cleanly.
Many guys who know will 'bounce' a mag on its' butt to be sure the ammo jumps in the mag, and with that little slack the ammo, spring, and follower all seat where they need to be.
In my older mags, I can run full load outs. These usually sit with a full load for a week or 12

in the closet in a box and this helps seat the spring.
DPMS/ Bushmaster magazines are notorious for not being able to handle their full load out, but they work very well when you load back by 1. So, on their 30, I run 28. On their 19 (Not 20) I run 18, and have zero issues.