PeteM, that .410 gun in the picture you posted looks a bit 'off face' to say the least. I would think that it would need the firing pins lengthened in order to reach the primer!

I have a little Belgian double with proof marks indicating 63mm. chambers, which is about 2 1/4". I have an aluminium cartridge of Belgian origin of this size. I usually use the 2" ammo in it. Lagopus.....