Bell's articles do indicate that there's often a mild reduction in pressure if the forcing cones are lengthened. Lengthening the chamber does nothing to make the gun stronger, and American factory 2 3/4" shells, in almost all cases, are higher pressure than factory 2 1/2" shells. The problem, however, is not the length of the hull in and of itself, but rather the pressure generated. You can reload, using American 2 3/4" hulls, and shoot them in short-chambered guns without any worries--IF you keep the pressure within the parameters for which the gun was originally designed. Prior to the advent of the 2 3/4" hull as "standard"--and standardization was not arrived at all at once by all manufacturers, nor in all gauges--pressure for the old short shells was about 1,000 psi or so lower than in the then-new 2 3/4" shells. It's likely that both a Sterlingworth and a Model 12 with factory short chambers are strong enough to handle that additional pressure, even though they were not designed to do so. But you will be kinder to your old gun if you stick to loads within the pressure parameters for which they were designed, either by shooting factory short shells or by reloading 2 3/4" hulls at lower pressures.
And with the Model 12, as mentioned above, there may well be ejection problems with 2 3/4" hulls.
