As Miller pointed out, weight is an important consideration. I've weighed several Flues 20's that are a bit over 6 pounds. On the other hand, there are others that weigh in that 5 1/2# range. And as best I can recall from research a former poster here had done, the issue with those very light Flues 20's often was not with the barrel, but rather the receiver. They had a tendency to crack right where the standing breech meets the water table.
Definitely a good idea to take careful measurements of any
Flues 20 that now has 2 3/4" chambers. More critical, again, on the lighter ones--because they had thinner barrel walls to produce that light weight. And removing an extra 1/4" to lengthen the chamber could be a very big deal.
If chambers have already been lengthened to 2 3/4", then reloading those hulls to low pressure should work every bit as well as shooting 2 1/2" shells at the same pressure. The longer hull won't give you an additional pressure increase, as it will (although that increase may not be more than a few hundred psi) if fired in a 2 1/2" chamber.
