For preference, I would always use a double (either rifle or slug shotgun) for boar - it can be a challenge to work a bolt sucessfully with a stroppy pig closing fast. I have also used a BAR but that can create its own problems.
I have used a 9.3 x 74 s/s (popular in France and elsewhere in Europe so good choice of ammunition) and a .375 s/s (make sure you bring whatever you will need), and both were fine. I felt that the 9.3 was plenty enough and the H&H was a bit much, but it does make you feel very safe.