Many people use hunting revolvers. Apparently boar hunting involves a lot of brush, where longer guns are at a disadvantage. They also involve close-range, fast-action shooting, and some danger, as boars have a bad temper and are known to charge. They also have a pretty heavy body structure.
In the articles I have read, a .44 Magnum seems to be popular, like a Ruger Redhawk or Blackhawk, loaded with a good hunting bullet. Powerful, with fast shooting ability.
Marlin also makes short-barreled big-bore lever-action rifles. You can get them in .44 Mag, .45-70, and .450 Marlin. A nice one is the M1895GS, which is what Marlin calls a "guide gun". It has a compact 18.5 inch barrel, 4+1 magazine capacity, comes in regular or stainless steel, and fires the potent .45-70 cartridge. With the Guide Gun concept, you can mount about halfway down a barrel a low-powered scope. This setup is generally considered very fast and accurate for close-to-medium range shots, because you can see around the scope to bring the gun on target, then quickly sight through the scope for the shot.
http://www.marlinfirearms.com/firearms/bigbore/1895GS.aspxIf you want the lighter (not light, but "lighter") .44 Mag, the gun holds 10 rounds with a 20" barrel.
http://www.marlinfirearms.com/Firearms/1894Centerfire/1894.aspxFor either gun you can buy the the scout scope mounting bracket for about $60, or just use the included iron sights.
Oh, and Ruger also makes a .44 Mag semiautomatic carbine. It also has a short barrel and 4+1 capacity, and it will shoot faster and easier then a lever action. It's called the Deerfield.
The muzzle energy of a rifled slug from a 12-gauge runs between 2400 and 3000 foot-pounds. A .45-70 cartridge runs about 2400 ft-lbs. A .44 Mag fired from a rifle runs about 1600 ft-lbs. A .44 Mag fired from revolver is about 750-850 ft-lbs.
By contract, your 9mm Walther can run about 350 ft-lbs or so. And they dont' really make any deep-penetrating hunting bullets for the 9mm because it's not really a good hunting cartridge.