It does count as a magazine with 10rd limit. The ban only exempts tubular devices designed only for 22 rimfire, such as the Marlin, Winchester, etc.
And semi-auto shotguns are affected by the ban in this way-
They can have only one out of four features:
1. Fixed mag over 5rd
2. Detachable mag
3. Pistol grip
4. Folding stock
Some of those are the same as rifles. But flash hiders, bayonet lugs, and grenade launchers are unrestricted on shotguns.
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