The T-34 is a tricky tank all things considered but you seem to have done an excellent job, especially with the diffuse texturing. My only question is in regards to the scratches on the turret? I don't see how they could have been caused but that may just be me.
Part of the problem with the gun barrel is that you're missing the taper - the diameter of the barrel at the base is considerably larger than that near the muzzle. The barrel should be off-center; in the top-view, notice that the dashed line down the center of the barrel is to the right of the cross at the center of the…