SRPRAKASAM:
Firstly, congratulations if the gun is yours. The Colindian was Lancaster's version of the Paradox gun, popularised by Holland & Holland. An internet search on 'Paradox Gun' or 'Rifled Choke Gun' will give you plenty of useful information. Even though your gun was made very differently it was intended for the same purpose, and to safely shoot the same ammunition. The intention was to pattern well with bird-shot, and shoot a solid lead bullet as accurately as an express rifle, from the same barrels!
The Colindian should shoot a good cylinder-pattern with #6 lead bird-shot in felt-wad shotshells, ie perhaps 40 to 45% in a 30-inch circle at 40 yards. Other shot sizes, and plastic wads, will certainly alter the results but it is difficult to predict the outcome. You will certainly have to test it yourself with SG or LG, as that would probably not have been tested or published at the time. Cartridge length will almost certainly be 2 1/2 inches, but the length should be marked on the barrel flats. Later guns may have been made with 2 3/4" chambers, just measure them. If not clearly marked 'Super Magnum' somewhere on the barrels or flats, then it isn't.
The projectile best suited to the Colindian is almost certainly the 'Fosbery'-style projectile, which is slightly cotton-reel shaped, and typically weighs 750gr as a solid, or perhaps 690gr if hollow-nosed. The nitro versions of the cartridge would have been loaded with chopped cordite, moddite, axite etc, which are unavailable commercially in the west today. Best modern loads are published in the second edition of Graeme Wright's "Shooting the British Double Rifle", Chapter 14: Paradox Guns. I strongly recommend you purchase a copy.
I could post some sample loads, but have no idea what powders are at your disposal in India.
The info posted above is offered in good faith, but there are no hard and fast rules in dealing with vintage British guns. If you were to post a digital photo of the barrel markings and proof-marks on the barrel flats, we could be a little more certain of some of the above points.
Nevertheless, I hope this helps in some way.
Almost forgot: Photos of Fosbery-style projectiles from my mold.
Several custom mold-makers in Australia and US carry this pattern. This one is from CBE (Cast Bullet Engineering).
