Marine... I have never heard this expression before. I searched the internet to see if t is an idiom or common expression, and I found NO results. In the extract, I agree, it doesn't make sense to write 'shoot boar and bear'. Perhaps you could write "one doesn't need a telescopic sight to shoot political targets in public OR to shoot targets that are in plain view (i.e., easily seen).

I actually read that book and saw the movie starring Peter O'Toole. He is trying to kill Hitler because of his own personal hatred of Hilter and the Nazis and not working as a goverment agent, so when they find him with that gun it is clear that he is an an assasin because those rifles scopes are specially made for that purpose ( and not hunting). Powerful scopes such as that are very expensive and can cost thousands of dollars ( much more than the cost of the rifle itself). Again it is the same as today the man who was on the French train with several machine guns said he only wanted to rob some people, when clearly his purpose was more than that.

I'm a U.S. native speaker. It's not an idiom as far as I know. I think the writer coined it and enjoys the similarity of the two words--it's a faint joke or pun. I'm not a hunter but I guess these are animals you get close to when you are hunting them. If I were translating it, I would not translate it literally--that is, I would not use the French words for "boar" and "bear." If you can think of two French words that are the names of two commonly hunted game animals that are hunted at close range, and that look or sound similar, you could use those and translate the pun as it were. Otherwise, I would just use something straightforward like "a game hunter doesn't need a sight like that." Is there an earlier passage in which the assassin claims to be a hunter of some specific kind of game?

It is not an idiom. I think maybe it refers to the fact thst the assassin used a rifle with a powerful scope (telescopic sight) that you would not usually use to hunt a bear or boar, so it means he was using it for illegal purposes (assassination) so that he could kill the person from a far away location ( a great distance from his target ) and not be detected. Sort of like you would say "well clearly he wasn't hunting rabbits with a Kalashnikov machine gun." if you found a suspect hiding in the woods with such a gun.