Got an email from Brownells because I had a pending order for some M&P 17rd mags it says they will no longer sell the following to CO.
"Rifle or pistol magazines with capacities exceeding 15 rounds
All shotgun magazine extension tubes
Removable shotgun magazines with capacities exceeding eight rounds"
The law clearly bars the first and last items, but in regards to tubular shotgun mags it says they must be under 28". Brownells sells a variety of shotgun extension tubes that when installed are less than 28" in total length. I pointed this out to Brownells but it sounds like they are going to keep this policy, I suspect because it is hard/impossible for them to know what the total length of the extension tube will be once installed (because they don't know what it will be installed on). Really sucks.