Hi,
I have just bought a German drilling (9.3x72R and 12-ga with 2 1/2" chambers). It's about 60 years old but in excellent condition with very tight action (like new). It also has the nitro proof mark on it.
1) Does anyone know who makes good 12-ga 2 1/2" shotshells? Do I have to use shotshells that are lighter loads with lower velocity and pressure? I don't know whether a drilling like this can use regular modern shotshells. Looks like it can but how do I know?

***I have a few 12 2 1/2 but many 16 2 1/2 and typically have used gamebore for both. http://www.gamebore.com/game-loads.asp#16-game-hunting . I have also heard that Polywad seems to work well and they have a better shot selection. You can also buy direct from them so they should be cheaper then Gamebore http://www.polywad-shotgun-shells.com/2-1-2-shotshells/ . When I bought the Gamebore, I belive it was ~ $95.00/Flat but that was before they decided to introduce distributorships (there are almost none in the US so you have to buy from some guy 5 States away and pay the shipping PLUS his markup) so it's almost not worth the effort. Expensive, yes, almost double that of what a flat of typical ESTATE target will run you BUT the pressure is kept low (around 8000) which is what these were typically made for. I have seen folks extend these to 2 3/4 but to me that is a turn off and ruins the integrity of the gun. Also, the higher pressure rounds have known to loosen some of the drillings (cant remember where I read this). This has not happened to me as I have always shot the low pressure 2 1/2. Think about it though, the Brittish and Germans, Austrians and others have shot with 2 1/2 for atleast a Century and have killed birds as large as Auerhahn's and other species just fine. I have hunted Pheasant with 16 2 1/2 #6 with my drillings in the Dakotas for the last few years and kill them just as dead (with one shot) as a guy with a 12 2 3/4

2) Should I lengthen the chambers to 2 3/4" so that I can have a better selection of ammos? Does that lower the value of the drilling?

My thoughts above. However, that's my thoughts and the gun is yours. You may opt to lengthen so it suits your needs. I would though test it with the 2 1/2 and see if it meets your needs.

3) The Sellier and Bellot 9.3x72R 193-gr SP ammos have a muzzle velocity of 1952 fps. It also uses flat-point bullets. The b.c. is pretty low. I am thinking about a muzzle velocity of around 2,200 fps with 200-gr Hawk round tip bulets. Do you think it's a good idea? Anyone tried that with success?

Have not tried it. The S&B seems to do the trick for me but I am not typically shooting a great distance with my 9.3x72r - - usually less then 150yds.

Please post pictures of your acquisition. I would love to see it. Can you share any data like maker, etc?