One of my friends has two guns with titanium parts: a S&W 329PD, a revolver with a titanium cylinder, and a Kimber Mountain Ascent, a rifle with a titanium bolt handle.(The pictures belove were downloaded from google)
Kimber Mountain Ascent
These are both relatively expensive, niche guns, built for a very specific purpose. Neither is any fun to shoot, due to their extremely low weight. Shooting more than a few rounds is painful for your wrist and/or shoulder. For a given caliber, a lightweight gun has significantly more recoil than a heavier gun, due to the laws of inertia (your wrist/shoulder absorbs the recoil, not the weight of the gun).
The 329PD is a .44 Magnum pistol, designed for personal protection in grizzly bear country. It's lightweight enough to carry with you at all times, but powerful enough (just barely :-) to kill a charging grizzly. You hope to never have to shoot it, so you don't mind the wrist-snapping recoil.
The Kimber rifle is optimized for elk hunting in high altitude mountains. Over 20 miles/day or more of hiking, with 1000s of feet of elevation gain, every ounce you can save on your back adds up. If you're lucky, you'll take 1 or 2 shots, and won't mind the shoulder pain (and barrel that would overheat with 3 comsecutive shots).
So, there are titanium gun parts, but the average guy plinking in his back yard or at the range, or hunting deer within an hour's hike of his truck, doesn't need or want a titanium gun.
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