I would fall it for the wider then 3 feet part due to the curve and all
You couldnt fail it for that sir, as it is only 3 feet wide when held as a shield,
You would really have to bend some interpretations and rules to fail it for being "wider than 3 feet"
IF it was comepletely torn apart and the foam unfurled you could though...
And if you did completly unfurl it you would have a solid piece of foam that was 3 feet wide, and chin to ankle,
When I wear the barrel shield I am wearing it on its side. because a shield can be worn in any manner. when i hold it the barrel is exactly 3 feet wide and if you measure along the curvature, which is actually top to bottom it is a little under 5 feet, which is chin to ankle
So even if you did measure it along the arc, it is still a legal shield - per the rules