I picked up a couple of the translucent smoke 20 rounds magazines, which sure look impressive.  But I can't get them to lock into anything I own  - they just won't engage the catch.  PMags work just fine, so I'm guessing it's an issue with the Lancer mags?  

Is this common?  Did I make a mistake buying these?  These aren't cheap mags either...

Asked this in the AR15 Magazines forum as well, but maybe that wasn't the right spot, since these are AR10/SR25 magazines really

What brand of lower receiver?

PSA

not yet, figured I'd start here.  I sent a note to my retailer about it, but haven't heard back yet.

Ok, I sent them a note, we'll see what they say.

In the meantime, I was able to test where the magazine is hanging up.  Turns out, the mag-lock hole is too low in the magazine.  The feed lips top out and compress, before the hole clears to where the catch is.  This is occuring in two rifles, so I'm thinking a magazine defect.

I can either take a file or dremel and basically open up the top of the hole, or just try to return them.  At $50/mag - I'm hesitant to just alter them myself right now.  Kind of dissapointed they didn't make them right.  

PMags work fine, and aren't even compressing the lips - I can fit a piece of paper in there even.  But not so with the Lancer L7s.

Which apparently is only half of it.  I was able to get it to lock into a PSA ar by camming the upper off, so that lips topping out wouldnt be an issue.  Then cam the upper down on it while it was locked into place.  That made thr L7 now able to pop in and out (probably by compressing the feed lips so that's... maybe great, or not)

But it still won't lock into the other at all.  I think the top of the hole is short.

PMag hole size = 0.258 Inch
Lander hole size = 0.251"

I think it's really that close.  I bet it has to be soemthing like 0.255

Are both rifles PSA rifles? Of the 30 or more Lancer mags I have in rotation ive never had an issue with my 2 Colts, Geissele, or 3 Aero builds. I dont see them changing the location recesses on the exterior of the mags.

Yea, I just used the stubby end of a bit sticking out no farther then the bottom depth of the hole. And just slightly worked open the top of the hole.  Now the mags insert and function in everything just fine.   Annoyoing I had to do that.  Note that these are the .308 mags, not the 5.56 mags.