I think the main effect would be that the muzzle velocity will be lower.
This is because the expanding gases of the gun power will have a shorter distance along which they push the bullet. Once the back of the bullet emerges out the short side of the barrel, the gases will escape and depressurize, loosing the ability to keep pushing the bullet.
The main effects of the lower muzzle velocity will be lowering the range of the bullet (i.e. it will fall to the ground after traveling a shorter horizontal distance), and lowering the penetration capability of the bullet.
I don't think this will make the gun fire with an angle.