You need the smaller/thinner base plate on the 15-round mags to make the adapters work. The thicker plastic ones will not allow for the attachment of the grip adapter to the mag. I have found that the mags don't stick out all that far, and that the adapter makes slamming one of them home a scary moment, and a "tea with the queen" grip is required to avoid blood blisters.
I carry the 6906 with a 12 round mag in it, and stick the 15 or 17 round mags in a carrier.
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