Algodoo

I hope you don't mind a few bulky invisible parts, JaXNash. No way to match starting speeds with the gun, so it's either bulky or doesn't work right when moving fast. Doesn't have to be heavy, though; you can reduce the density of the gun with very little behavior change. Sorry there are no pens, but spawning complex objects is complex, and way harder if it's going to be fired at multiple angles.

F to fire, L to delete existing bullets if it gets laggy.
Alter the rate of fire by changing the hinge speed on the little ball in the back of the barrel. Bullet properties can be altered in the onCollide of that ball. Currently set for maximum rendering simplicity and approximate properties of steel. Also set to spin wildly mostly because I felt like it, and also to add a little extra punch and unpredictability.

Edit: made the design more robust, and hopefully, neither of the springs are going to be corrupted this time.

Edit 2: more powerful, perhaps a bit more resistant to rapid movement. hidden parts are longer, though.