  Recently proposed Feynman-diagram (FD) gauge propagator for massless and
massive gauge bosons is obtained from a light-cone (LC) gauge propagator, by
choosing the gauge vector along the opposite direction of the gauge boson
three-momentum. We implement a general LC gauge propagator for all the gauge
bosons of the Standard Model (SM) in the HELicity Amplitude Subroutines (HELAS)
codes, such that all the SM helicity amplitudes can be evaluated at the tree
level in the LC gauge by using MadGraph. We confirm that our numerical codes
produce physical helicity amplitudes which are consistent among all gauge
choices. We then study interference patterns among Feynman amplitudes, for a
few $2\to3$ scattering processes in QED and QCD, and the process
$\gamma\gamma\to W^+W^-$ followed by the $W^\pm$ decays. We find that in a
generic LC gauge, where all the gauge boson propagators share a common gauge
vector, we cannot remove the off-shell current components which grow with their
energy systematically from all the Feynman amplitudes in $2\to3$ processes. On
the other hand, the $5\times5$ LC gauge propagator for the weak bosons removes
components which grow with energy due to the longitudinal polarization mode of
the external bi-fermion currents, and hence can give $2\to2$ weak boson
scattering amplitudes which are free from subtle cancellation at high energies.
The particular choice of the FD gauge vector has advantages over generic LC
gauge, not only because all the terms which grow with energy of off-shell and
on-shell currents are removed systematically from all the diagrams, but also
because no artificial gauge vector direction dependence of individual
amplitudes appears.
