I bought a bunch of the original L5 mags right when Sandy Hook happened in 2012. What I have discovered since then is that while they work perfectly fine in most rifles,...some rifles not originally designed for AR15 mags but outfitted with an adapter to take AR15 mags can have issues with L5s (and perhaps others as well).
For instance, I have had two 5.56 AKs modified to take AR15 mags;...a Zastava M90NP with the polymer factory-added mag well, and an Arsenal SLR106F that I retrofitted with a Canis Design Group adapter. Each rifle worked perfectly with Magpul mags and aluminum GI mags, but the Arsenal had trouble letting the L5s fully seat and lock. The issue is that the L5 stamped steel upper portion of the mag had corners that were just ever so slightly protruding out beyond the dimensions of the GI or Magpul mags. Barely a single millimeter. Not enough to cause any trouble in any AR15 or Robinson XCR or Beretta ARX or SIG 556 or other rifle designed for GI mags. But AK adaptations are internally very tight and cramped, and the conversions had obviously been engineered using the most likely mags as mandrels around which to engineer them.
I still love my L5s and they are my preferred mags to use,...just not in my Arsenal AK. For that one, I reserve my beat up aluminum and Magpul mags.