I am thinking AR width so any safety and trigger group could be used.
The AR stuff is such a huge yawn. I was at the SHOT show and there was nothing that jumped out. It felt like European vacation when they were stuck in the roundabout, except this time it was "Hey look, another AR"
Well, there was one AR that had replaceable mag wells so you could use all kinds of different mags. From pistol to AK.

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