Ummm... NO. And fixed breech guns. Sheesh. I'm very familiar with Mr. Bell's articles. The chamber and forcing cones are NEVER EVER shorter than the shell. It would create catastrophic pressures.
Using 2 3/4 inch shells in short chambers has been shown to only "slightly" raise the chamber pressure. Sherman Bell, in article titled "Finding Out for Myself" Part V "Long Shells in Short Chambers" (Double Gun Journal, Winter 2001) said that with loads that are sensible in a light 2 1/2 inch gun, he saw no dangerous pressure levels produced. He said there was no reason, related to safety, to lengthen an original 2 1/2 inch chambered gun to shoot 2 3/4 shells, as long as the pressure of the 2 3/4 inch load you use is the same as the pressure of a 2 1/2 inch load.
However, recoil may be significantly increased
, especially in a 20 gauge 2 1/2 inch chamber if the case mouth extends into the forcing cone, or even worse, the bore. Remember that the stocks of L.C. Smith shotguns are somewhat delicate, and a cracked stock may result from more recoil.
And yes, many American shotguns the 2.5" chambered guns measure 2 3/8" to the start of the forcing cones. This was done to compensate for poor gas seal. 2 3/4" guns often were shorter to start of cone for the same reason.
Knowing the pressure spec from SAAMI for 12 gauge is 11,500psi. knowing that tests have shown minimal pressure increase and that opening into forcing cones only increases RECOIL the big issue is ejecting the shell from a repeater.