What is the purpose of this magazine cover...

[frontier737]

....shown here http://gunblast.com/images/Ruger-SR556/DSC07756.JPG

If it's only to keep debris from entering a stored magazine why are they not more common?

[jeremy155rr]

its 2 fold, to keep dust and debris out, and to keep the pressure from the fully loaded (stored) mag from deforming the feed lips (pushing them outward). thats from my understanding anyways.

[frontier737]

Thanks!

[crashgordan]

yep... with the quality of the springs in these Pmags, you can load em, snap the cover on, and it pushes the rounds down into the mag just enough to take all the pressure off of the feed lips. in theory anyway, you can leave them loaded forever and when you pop the top off, they are ready to roll. no gradually stretched/out of spec feed lips. have several of the all generations of pmags, and i have never been disappointed. incredible mags.