Here's a piece on the .303 double rifle from G.T.Teasdale ca1900...
"...In the Westley Richards make
the .256 is rifled to right and the .303 bore is rifled to left,
their .303 bore rifle is made on the Enh'eld five-groove system,
and the depth of the grooves is .004. It has about one turn
in ten inches to the left, and with the cartridge and load they
recommend for sporting purposes gives a muzzle velocity of
about 2,000 feet per second [37 grains Rifleite or Cordite,
2 1 5 grains bullet]. The sporting bullet they supply is of the
same weight — a great point in our opinion, and although one
is hollow in front and the other is not, there is no trajectory
difference in practice, whatever the experts may say..."
This might give you something to think about in regard to period loads for Brit double rifles built and regulated for the available .303 Rifelite sporting ammunition.
Again, enjoy the rifle!
--Tinker
