The 16 bore has come in various sizes throughout the years (2 7/16, 2 9/16, 2 3/8, 2 � ). The simple answer is yes, you can lengthen those chambers (best to just lengthen the forcing cone), and typically it�s not heresy. European rules are any work done to the barrels will require a re-proof. If you have the forcing cone lengthened and then send it off to Europe for re-proofing, that will ensure you maintain all the value. But here in the states, not everyone is much concerned about proof.
Shooting longer shells in shorter chambers has been proven to not increase chamber pressure by any degree that could be considered dangerous (typically 1,000psi is about average). In some cases you can see poor patterning, and if that�s the case, then lengthening the forcing cone is in order if you desire to use only modern shells. Otherwise, you can buy �vintage� type shells from companies like Poly Wad and you should expect perfect patterning. If the patterning is good with modern shells, then all is good and don�t bother with anything.
Most 16 bore drillings are choked full and full, which you may find desirable or may not. I personally would want the chokes opened up to IC in both or perhaps IC and Mod if I expected to do a good deal of pheasant hunting or such larger birds. With modern ammunition that uses plastic wads, most guns are significantly over-choked and I think you�ll find your full and full will pattern more like turkey chokes with modern ammo. For further advise, or if you�re considering any work for your drilling, I would recommend Nick Tooth, formerly of Purdey�s, and no one else:
http://www.nicholastooth.com/ 
