[gun with unlimited bullets]

No.

Behind the first bullet, there always is a combination of bullets which contains one that will reach the first one.

what you said is wrong. For your understanding, what if first bullet has velocity 1m/s, then it will reach infinity right.

[More random numbers]

.625

Yeah i gave a crude - well approximate answer.

The answer will be 2*(x/4) + 2*(x/4²) +2*(x/4³)+ ..... where here x=1, so the answer is 2/3=.666

[More random numbers]

.625