If you really want to know the true chamber length, you have to do a visual with a machinist's scale and proper light. Another fairly accurate way is to actually take a micrometer reading of the entire chamber to the bore, an eighth of an inch at a time. A hard chamber gauge can easily give a 1/4 inch inaccurate reading in a slightly oversized factory chamber. Your chamber could be a perfectly correct 2 3/4" chamber when properly measured.