The total momentum is unchanged according to the law of conservation of momentum. When the gun is fired, the bullet gains a high velocity forward (positive velocity), and that velocity multiplied by its mass is the momentum the bullet gains. Therefore, the gun must gain a momentum backwards to cancel out that momentum forward, so the gun recoils back with a negative velocity.
Using the same antenna. Now he doesn't know if it was the antenna that caused the change in reception. so he wasn't only measuring the reception in his house he was measuring it based on different antennas
