I have been hunting Chital deer, Kakar deer and jungle boars quite a bit, by employing buckshot. I personally prefer a 12 Gauge 3 "Magnum sidelock ejector side by side, which was built in 1932 by Janssen Freres in Belgium. The barrels are 32 " long and both barrels are improved cylinder Choke.
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I used to prefer Sellier & Bellot "Red Hare" paper cased 2.75 " 35 gram eight pellet LG shells in the past. Each LG pellet has a diameter of 9.14mm and is the equivalent of American OOO Buck. Unfortunately, Sellier & Bellot discontinued manufacturing shotgun shells loaded with any shot size larger than SG/OO Buck a few years ago.
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I currently use Federal Premium 3 " Magnum Copper plated ten pellet OOO Buck shells.
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In regards to chokes, this is a very technical subject area. Buckshot which is unbuffered and uses a fiber wad, should be used only in open choked shotguns (such as cylinder or improved cylinder). Otherwise the tight chokes will cause the buckshot pellets to get deformed while passing through the barrel and your pattern will be erratic. Buckshot which is buffered and protected by a plastic wad can be used in even tightly choked shotguns (such as full or extra full), often with good results (i.e tight patterns).
I personally prefer side by side shotguns, but any format (be it over & under, pump action, semi automatic or single barrel) may be used for safely shooting buckshot. The platform is of no consequence. But side by side or over & under shotguns are more likely to have their barrel soldering eventually get damaged if heavy buckshot is repeatedly fired through them (especially the delicately built English shotguns). For this reason, double barreled shotguns (which you intend to use for shooting buckshot) should always have very strong walls.
In my experience, the bulk of the manufacturers of shotguns discourage using OOO buckshot in shotgun barrels which are choked tighter than improved cylinder. The reason being, that they believe that large buckshot through a tightly choked shotgun barrel will eventually cause a barrel bulge. These manufacturers include:
Winchester
Browning
Fausti
Rizzini
AyA
Grulla Armas
Armas Garbi
Ignacio Zubillaga
Merkel
Savage
Huglu
Baikal
Westley Richards
Watson Brothers
A. A. Brown
Verney Carron
Lebeau Courally (although they might not be manufacturing any new firearms, anyway)
Birmingham proof master, Sam Perry believes that shotguns which are intended for use with heavy Buckshot ... Should not be choked tighter than quarter choke with a good length of the cone leading in.
To the best of my knowledge... Only Beretta, Benelli and Remington (and the original Czech owned BRNO) encourage the use of OOO Buck in fully choked shotguns.
You definitely don't need a 12 Gauge with 3.5 " Super Magnum chambers. The recoil is absolutely beastly. But if you must get one, then the Benelli Super Black Eagle 3 is the greatest 3.5" Super Magnum shotgun ever to be made.
3 "Magnum chambers are far more reasonable and practical. And I've never found 3 " Magnum shells to be lacking in any department.