Update: Magpul PMAGs
I’ve made it to the range now with the magazines that I mentioned here.
All three functioned fine. I loaded them up with 30 rounds green-tip .223 surplus with nary a hitch. The weather was pushing 80 degrees and the rifle was a Bushmaster Dissipator. On the 3rd and final magazine I used it as for my fore-end grip and made sure to pull back on it good and hard. No idea why, just thought that it might trip it up.
Nope. No problems.
I think I just might go crush every other AR-15 magazine I’ve got in the basement. I don’t think I’ll be needing them anymore.