The gun on it was the same NS-37 gun fitted to the IL2. Other versions of the Yak-9T had about every kind of Soviet cannon ever mounted on an aircraft, including a 45mm one. One version even had an internal bomb bay for PTAB bomblets. The 9T was effective against tanks in the war, even more so against shipping.
This is a great plane for the defense. Its small, nimble, packs a huge punch, and sheds "E" well enough for those fast landings under duress. Best of all its a great perk farmer.
I like to take one up at a base under attack, spin around the base in the grass until I can maintain 200 mph to 250 mph, and pick off targets of opportunity. Using 3 shot bursts of the big cannon. Its great against PTs or LVTs, or M3s. Even against flak I you have enough Alt you can come down with enough energy to kill the flak gunners.
And as has been mentioned, its a great bomber killer. I just started flying it and am wondering why I waited so long. Anyway these are my impressions of the plane.