Gauss guns, those roller bearing ball shooting magnetic toys, fun as they are, suffer from the limitation of having to re-configure all the elements for each shot.
And, too strong Neodymium magnets may be destroyed by those hardened steel spheres, because they are directly impacted.
So this is my solution, first, the magnet is out of line with the balls, and second, the balls have a no-return latch, so, all you have to do, to re-set the thing, is to move the balls forward.
Neat, huh? :)
And the Latch uses a small steel or iron part, a bolt, etc. in that hole, there, so you don't need to add any spring to keep the latch down, see the second picture:
Don't want to make it?
You can have it 3D printed:
Step 1: Cut a Tube
Just cut a tube, where the steel balls can go trough, 5 times their diameter is enough length.
Step 2: Cut the Latch
Then, cut the tube, so a portion of it can act as a latch, flexing up and down.
Step 3: Drill a Hole
A Screw has to act as the latch's lip, so you drill a hole where it will be fit.
Step 4: Fit the Screw
It better be a brass screw, or you'll have a hard time if you ever have to re-fit it, after the strong magnet is just 2 Cm below...Then, nothing to it, fit it into the hole.
I've filed into a ramp, the sliding down bit towards the entering ball.
Step 5: Reinforce It, If Needed
This plastic is too flexible, so I've added a metal plate, and held it into place with these nylon strap-on ties.
Step 6: Glue the Magnet
Glue the Magnet not exactly below where the attracted ball will be, but a bit forward, so the attracted ball will still strike the one held back by the latch..
Step 7: Fix the Cannon
Might be a swivel, so you fix the cannon on a base and rotate it up or down, might be another way to fix the thing, I just did it this way.
Step 8: Place the Balls
Place the balls in front of the latch, 2 are enough to make it work.
Step 9: Shoot It!
As you place a ball near the rear end of the cannon, the thing will jump and strike the other 2, and so the front one will shoot out like ,well, a cannon ball. :)
Step 10: Re-set
Just push the ball forward, so there's 2 balls, again, at the front of the latch.
Step 11: Repeat
Shoot the thing to your heart's content!