It tells you what it's chambered for. A good rule of thumb is if it's a 2-3/4" chamber, the longest round you can put into it is 2-3/4"
If you have a 3" chamber, you can chamber 3" rounds as well as 2-3/4". 3.5" chamber can chamber 3.5", 3", and 2-3/4". I'm sure you get the idea. I bet you could cram a 3" in the 2-3/4" chamber through the ejection port, but you would easily create an unsafe situation doing so (and it may be wedged past the forcing cone as well). And good luck getting it back out
