Per Sherman Bell's research on long hulls in shorter chambers, simply lengthening the forcing cone without lengthening the chamber will negate much of the pressure increase (which is typically less than 1,000 psi) resulting from the slightly longer hull.
I have a modern British 16ga (Webley & Scott 700) with 2 1/2" chambers. The chambers are factory, but it's obvious that someone had the cones lengthened. I shoot 2 3/4" shells, reloaded to low pressure and with 7/8 oz shot, in that gun. They work well. For heavier hunting loads--and I won't go any heavier than 1 oz anyway--I'll use RST's.
