I'd expect almost any gun -- whether polymer-framed or steel-framed -- buried unprotected in moist soil or stuck away in a drawer in humid conditions, maybe wrapped in cloth, will be a problem weapon after an extended stay there. For while the polymer might not be harmed, any steel inside that frame that gets moist will corrode and degrade -- and depending on conditions, even aluminum alloys can corrode. All of the springs in the gun, which are critical to function -- recoil, striker, hammer, magazine -- are likely to fare poorly if kept stored in such bad conditions.
As I understand it -- imperfectly, to be sure -- aluminum can't rust (as it's condition that affects iron or steel) but it can -- under the right (or wrong) conditions -- corrode. Depending on the cause of the corrosion, and whether the aluminum is in contact with other metals or the by-products of their deterioration, an electro-chemical process can take place between the different metals which can cause even more damage to the aluminum.
This past year, I picked up a beautiful old (full-size) Llama 9mm 1911 imported by Stoeger, back when Stoeger really watched quality control; it was the second such Llama I've owned. The first was a tack driver after a broken part was replaced and the gun lightly tuned. I later sold it and found myself regretting the sale. I found a replacement locally.
The replacement looked like it just came from the factory, but it had a very heavy trigger, and the grip safety was stubborn and didn't always work as it should. While I've done some home-gunsmithing on S&Ws and CZ, and BHPs, I've never had much to do with 1911s -- they are a bit of a mystery to me. I gave the Llama to my gunsmith to get right.
The gunsmith later said it was badly rusted internally; happily, he was able to clean it up pretty well. The trigger is still heavy -- a new project for him, one of these days? -- but now crisp; the grip safety works as it should, and the rust is no longer a problem.
That gun was NOT buried, seemed to have been cared for, came to me in it's original box, with all papers, and looked like it was ready to go on display somewhere.
I mentioned my Llama only to make the point that metal -- even metal that was seemingly well cared for -- is no guarantee of long-term functionality unless its truly well-maintained. The same would appear to be true of polymer-framed guns.
The plastic used in gun frames, nowadays, seems to be almost as durable as steel or alloy, but we won't really know about long-term durability for a long time. But it looks as though, with proper care (and protection -- like not leaving the gun out on a hot rock in Death Valley for several years -- it's going to hang around without crumbling to dust.
The other point nobody ever addresses in these arguments about the relative longevity of metal or polymer guns is PARTS!! Repair parts may be a much bigger problem than frame materials.
It's difficult to get parts for many guns that are over 30-40 years old, now, and it won't be better 70 years from now. Ruger doesn't have parts for some of their relatively new guns, some Third Generation S&W parts are hard to find from S&W, and I'm sure the older SIGs are going to be a problem one of these days; parts for some of their NEWER guns can require long waiting periods; what's going to happen 30-40 years from now, if you need THOSE parts? I had a badly damaged BHP barrel a while back, and couldn't find a factory barrel -- and had I found it, the barrel would have cost me $400+. I got an after-market barrel; don't know if those companies will still be in business 20-30 years from now.
On the other hand -- I've seen they've made a whole 1911 using a 3D printer. Maybe that'll be the way we get gun parts in the next couple of decades... If technically competent, maybe we can just print them ourselves?
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