Product review (4 stars): One of the "big box" stores had a similar heavy duty cap which had a longer duct which might work better for going through outside walls with 2x6 construction
This worked well to vent a new over the range microwave to the outside. The horizontal dimension of the duct on this is actually about 10.25 inches so that when it mates with a standard 3x10 duct from the inside there is a gap which I caulked with high temp silicone to prevent air leakage. However, the extra .25 inch makes it easier to place the cap duct over the interior duct. One of the "big box" stores had a similar heavy duty cap which had a longer duct which might work better for going through outside walls with 2x6 construction, but the Broan 639 cap has far better bonded paint, a good damper, and a screen to keep critters out. So, for the Broan cap to fit, I needed to first add a short piece of standard 3x10 duct to the back of the microwave before installation. Overall, this Broan cap was superior.