Life, liberty and defending that freedom. If you could only have one hand gun for self defence. What measures would you take to ensure the longevity of that firearm. For me I would choose a full frame 9mm Glock, like a 17... or 18. Glock because it's ugly as sin, but ingenious in its design and reliability. As far as the ballistics choice, 9mm is very common and easy to obtain. Full frame because when the defection hits the oscillation and adrenaline kicks in I'd rather have every bit of help controlling my follow up shots that I can. The question is what measures can be taken to reduce the stresses exhorted to the weapon during operation. IE changing the spring to reduce the slide impact and installing slide buffers for frame protection, running lighter powder loads and grain ammo to reduce recoil, installing longer stainless steel anti corrosion treated barrels, etc. Remember one gun. That's it. You need to install every preemptive measure you can think of to keep that tool operational.