ok the myth is that if you
made a thing that gently
held a gun and shot it
(black powder gun ) it
would fly back 5 feet
and the bulet would not
go as far! cool!

Uhm....what?????

I think what togiman is trying to say is ...

the team builds a rig that holds a gun properly. they shoot the gun, then measure how far the bullet travels.

now adjust the rig to barely hold the gun. they shoot the gun a second time and see how far the bullet travels.

Not really a myth because every action has an equal and opposite reaction.

Because the force that would propel the bullet forward is now distributed to the gun as well.
The force now recoils and will send the gun backward. So the bullet wont travel as far forward.

The slide on a semi-auto does moves to absorb the enrgy of recoil and as far as i know this has no advers affects on ballistics.

the slide on a semi auto moves to eject the empty shell casing and chamber the next round. any dampening of recoil is a side effect and usually minimal. (I had a .22 pistol that the action of the slide actually INCREASED the percieved recoil)

a semi auto does have a very slightly lower muzzle velocity than non-self-loading rifle firing the same cartridge, however, the difference is very slight. (however, you will notice that competitive marksmen do not often choose to use self-loading rifles)

With long arms in general, even relatively low-velocity black-powder arms, the bullet leaves the muzzle before recoil overcomes the inertia of the weapon itself.

If you suspended the weapon on wires that allowed it to swing freely, it would have no effect whatever on bullet velocity.