Let's say I cut off the end of a gun barrel at a 45° angle: What would the effect be on the trajectory of a bullet fired through that barrel?
Would the bullet be less stable (I guess)? Would it make the gun fire with an angle, and would that be toward the "small" end?
Suppose you make the cut so that the slant is downward – i.e., the top part of the barrel is shortest, the bottom is longest. The bullet is driven down the barrel by the pressure of propellant gases on its base. As the base of the bullet leaves the barrel the propellant gases begin to act asymmetrically on the base. If the bullet weren't stabilized you might expect this asymmetry to introduce an overturning moment.
However, bullets are spin stabilized and so in practice no such effect is observable. In fact a slant cut like you describe is the simplest form of "muzzle brake" and has been used on many rifle designs: Its benefit (as with all muzzle brakes) is to divert some of the muzzle blast from the direction of the bullet, reducing the recoil force on that axis, and in particular offsetting "muzzle rise" associated with most small arms (which discharge from an axis above their center of mass).
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.
