In my case, I just have a bunch of the M2 non windowed mags just sitting here so I decided to try a few.
I already have 13-14 of the C-p and ASC mags that worked well for me, but I despise stainless mags.
I only have 6-8 of the windowed M2 Pmags in my stash, they have all been modified already.
No one disputes that the windowed mags work well, there has to be a simple reason why they work, yet the non windowed do not- it's simple physics.
And, it's not like we are rendering these mags useless.
Any mags I purchase (at this point) will be M2 windowed mags, but I would still prefer to use what I already have here- as long as the results are good.
Edit to add-->> I just pulled out a couple boxes of mags from storage, on top of one were 7 more windowed M2 Pmags (and a bunch of other series pmags). Will dig deeper...
Great find