Algodoo

unstable springs i think.

3/5

There are a couple of things hurting the accuracy of the gun. First is that it uses one super-bouncy thing to whack another super-bouncy thing that shoots the projectile. The more steps there are, the more randomness. The bouncy block has a bit of wiggle room in its track, too. The track also has default bounciness, where zero bounciness would be best.
The swinging arm that fires the gun is on a "loose" part. It's held there pretty tightly, but it still moves when the arm swings. More randomness, and occasionally it sort of breaks itself. Also the friction on it screws it up a bit too. It would knock the blue part more straightly if it didn't have any friction. The blue part is a polygon, too. That's never good for smooth, even interaction.
The barrel's good enough, but again zero bounciness would help. The recoil system fits really nicely, though.