coaxial cable

I need to know if the wire braid for coaxial cable is supposed to touch the o inside metal piece that screws on rg 6. tip obviously not the center conductor or if i trim as much of it as possible i have a coax to bnc connector and is it possible to ruin a bnc to coaxial by screwing in an...

Hello, How do i calculate impedance matching of RG58 cable connected to both a transmitter and receiver. Thanks