I've used K&S for my custom barrels. Had to extend the 120mm Leo barrel a bit for the Black Eagle build. You should be able to find This at Hardware stores like ACE.

BTW found the t-34/100 reference:

"At the end of the war, attempts were made to improve the tank's armament. In 1945, prototype T-34-100 tanks, with the turret ring widened to 1,700mm and armed with 100 mm LB-1 and D-10T guns, underwent field trials. This project had little purpose in so far as the new T-54 Main Battle Tank with the 100 mm D-10T gun had already been tested and recommended for service."

From http://english.battlefield.ru/t-34-85.html

Pic here:

http://www.ww2incolor.com/soviet-union/t34_100_02.html;)