A .243 will work fine though on a large boar you will want to make a head shot or neck shot because of the thick shield over their shoulders. We've shot boars with a shield up to 2" thick. At 50 yards with a .35 reminton 200 grain bullet it stopped the bullet on the far side and didn't penetrate completely. When we cleaned it we counted at least 6 other scars on it shield where it had been shot and not killed because the bullets didn't penetrate the shield.