I think this has been debated before with no real conclusion. Some say yes and some say no. Just grab a dowel rod the same size as the chamber and insert it until it stops. Then measure the length,. You might have to wrap tape around the dowel to get it closer to the size of the chamber. Or cut a strip of an old credit card the width of the chamber and insert it until it stops.
I went down and measured my 84 20 gauge and I get 2 3/4".
It is interesting though that I have an early model 840 that is just marked 20 gauge on the barrel like the 84. It too is 2 3/4". I also have a later model 840 where all the markings are on the barrel and it states 2 3/4 or 3 inches. Measuring the chambers you can easily see the difference on the dowel compared to the other 2.