It's usually pretty tricky measuring chamber length with a tape measure... too flexible. It's even tough with a 6" steel scale because there usually isn't a well defined step between the chamber and bore, but rather a taper. You didn't say what gauge gun your Flues is, but I don't think any of the 12, 16, or 20 ga. Flues had 3" chambers. Many were shorter than 2 3/4". Remember that a 2 3/4" shell is that length after firing and the chamber should accomodate the shell after the crimp is opened on firing. Have your gunsmith accurately measure the chamber length before he repairs the broken firing pin. If someone reamed short chambers out to 3" in a Flues, I don't think I'd want to shoot it anyway. I do have a friend who used to routinely shoot 3" magnum shells in a 20 ga. flues and got away with it. I also have two sets of 20 ga. and one 16 ga. flues barrels that have one tube badly split about where your left hand would be. I don't know what caused these mishaps or if any injury resulted, but I can see that the barrels are pretty thin at that point.
