Apologies Madcow, I wasn’t referring to your product but to the multitude of the ones available from the USA. Like the Taran Tactical Mag bases. Yes, they add a few rounds more to unpinned magazines, but to pinned mags they just add weight and don’t actually add any increased magazine capacity. There is no real rhyme or reason to CBSA’s thought process or even the application of common sense
The issue is that it is not as common as the name suggests.