  The Bargmann-Wigner (BW) scalar product is a particular case of a larger
class of scalar products parametrized by a family of world-vectors. The choice
of null and $p$-dependent world-vectors leads to BW amplitudes which behave as
local $SU(2)$ spinors (BW-spinors) if {\it passive\/} transformations are
concerned. The choice of null directions leads to a simplified formalism which
allows for an application of ordinary, manifestly covariant spinor techniques
in the context of infinite dimensional unitary representations of the
Poincar\'e group.
