No, you don't know the Walther GSP500 yet, even though the brand-new target pistol from Ulm shown at IWA can't deny its ancestor, the Walther GSP, either from its silhouette or its name. The classic Walther GSP, introduced back in 1968, is considered the world's best-selling competition pistol, with more than 150,000 made in .22 Long Rifle and .32 Smith & Wesson long calibers and countless …