  We consider the decay $\pi^- \to l \, \bar{\nu}_l$ ($l=e^-,\,\mu^-$) in the
presence of an arbitrary large uniform magnetic field, using the symmetric
gauge. The consequences of the axial symmetry of the problem and the issue of
angular momentum conservation are discussed in detail. In particular, we
analyze the projection of both the canonical and the mechanical total angular
momenta along the direction of the magnetic field. It is found that while the
former is conserved in the symmetric gauge, the latter is not conserved in both
the symmetric and Landau gauges. We derive an expression for the integrated
$\pi^- \to l \, \bar{\nu}_l$ width that coincides exactly with the one we
previously found using the Landau gauge, providing an explicit test of the
gauge independence of that result. Such an expression implies that for nonzero
magnetic fields the decay width does not vanish in the limit in which the
outgoing charged leptons can be considered as massless, i.e. it does not
exhibit the helicity suppression found in the case of no external field.
