It's my opinion that all of this has been covered in regards to polymer magazines... The conclusion there is that metal reinforcement is critical at feed lips and locking lugs -- without, polymer simply can't stand up to the abuse. Additionally, polymer is absolutely more sensitive to temperature changes. It cracks when cold and softens when hot. Take a look at at g36 issues for hot, or again, the polymer magazine tests when cold. There's all sort of info out there from Magpul's pmag feed lips spreading when loaded; hence, the "dust cover" to relieve pressure as well as various magazines decomposing when exposed to deet, etc.
And take a look at the best of polymer mags -- glock mags are metal lined, best of European AK polymer mags have metal reinforced lockign tabs and feed lips, lancer l5 mags have the same for ARs. Etc. Etc. Polymer by itself is inadequate. The interplay between polymer and metals is where optimal durability seems to occur.
The sl8 to g36 conversions are also a great learning opportunity as well -- and once cut, they have to use metal for reinforcement as once cut polymer can't be molecularly welded back to original strength like metals:
http://www.thefirearmblog.com/blog/2013/05/15/yourself-hk-g36-rifle/For polymer to be overall superior, the item must be:
1) designed from the start for polymer -- for polymer to have the same strength it absolutely needs more volume of material
2) not subject to severe temperatures -- plastics and plastic based polymers, just like glass, are "squishy" substances that are not completely either a liquid or a solid. Accordingly, they're much more sensitive to heat. Fibers such as fiberglass and other internal structures are often used to help strengthen, but they'll never 100% overcome the drawbacks of the physics of the basic polymer... Generally, makers will either bias towards strength/solid (also more fragile) or bouncy/liquid (less fragile but also less stable). Both have drawbacks/benefits...
3) Not be subject to severe stresses -- take a look at the Bushmaster carbon AR uppers and all the trouble they had with extremely dangerously breaking apart during firing...
So there are absolutely places where polymer can be used, but it's a tradeoff.
The primary gains from polymer are:
1) Ease/cost of manufacturing (substantially reducing costs and time), especially in regards to rapid R&D (even more so in the age of 3D printing -- polymer in weapons is sort of the major advancement that CAD -- computer aided design -- was for all types of manufacturing and CNC machining 20 or 30 years ago), though the materials and machining costs are reduced rapidly as well (polymers wear out metal machining at a much, much slower rate than does machining metals where specs need checked with much greater frequency)
2) Less weight -- lighter than steel, not lighter aluminum
3) Moisture ambivalent for the most part (I'd argue all guns will have metal in them, so the gains in this department are limited)
What I don't like about polymer are that unlike metals:
-you often don't see the item is failing/degrading until it cracks
-it simply can't take all the same abuse as metal and is much harder to repair when it fails -- you can beat dents out of metal mags/stamped receivers and reweld most types of metal to restore to 100% original strength and function... When polymer goes similiarly it's trash -- but softer polymers are good as recoil buffers to protect strong receivers from shock and external receivers to protect internal linings/stamped metal from dents, etc...
-metal engaging with polymer will more rapidly wear out polymer than equivalent metal -- guide rods w/ metal slides, polymer AR uppers with steel bolts, etc...
-generally it's occupying more space to accomplish the same function (and greater volume means more material means less weight savings -- the 5.56 Bren mags are larger than 7.62x39 VZ58 mags and weigh more for instance, one of the major complaints of that weapon system)
The parts I tend to replace with metal are anything related to fcg (believe I get more tactile feedback/vibrations from metal fcg parts than I do from polymer) or weapon ignition (the beretta storm carbines have polymer hammers and a bunch of other exceptionally important components in polymer -- to me this is simply beyond belief). And on pistols, I also replace guide rods with stainless steel ones whenever they come w/ polymer...
(I should also note that polymer by its very nature, is more reliable the less stresses being put on it. So while it may well work great with pistols and other guns firing the same ammo, when you put it in a rifle caliber weapon without redesigning the weapon's gas system to minimize recoil, bolt carrier, and other forces to a level that's coherent w/ polymer, failures will occur...
Lastly, I'll add the -- military and civilian firearms requirements are divergent. Due to field conditions of militaries, there's most definitely something to be said for polymer to entirely replace wood (days or weeks in jungles/swamps vs a half day when it's drizzling while sitting in an open hunting stand) -- there's less to be said for replacement of metal except for specific AOs like at sea, etc, where you are best served to minimize metal (but metal encased by polymer is a great protection/strenth option as well -- and modern metal coatings are so much better than the limited bluing, parkerization, chrome, paint, and stainless options available just a generation ago -- nanotech and similar that have benefitted polymer have benefitted metal coatings and metal alloys as well). For sportsmen, a wood stock that's glass bedded will meet 99% of your requirements satisfactorily, and since you don't have to carry several hundred rounds of ammo as well as any kit or food to sustain yourself for a certain number of days in the field w/o resupply, the few ounces in weight increase w/ wood over polymer on your rifle are negligible... And you can configure/balance your weapon too so that any add'l wood weight does not substantially affect pointing/target acquisition speed...
Again, the modern military rifle acquisition process has requirements of weight minimization and quick change multiple caliber capable with also a very strong bias to the cheapest weapon option... Accordingly, that's why from sidearms to rifles, polymer has dominated new weapons development as that's the only way weapons manufacturers could make a weapon that would be at a price point for mass adoption and armament -- from R&D to production. The weight tradeoffs you can see w/ many of the latest generation military rifles have lightweight/thin profile barrels, which when hot completely lose their capability for accurate fire as well as severely restricts the weapon's capability for extended full auto fire...
In the consumer market, everyone's requirements will differ, so I'd encourage you to do your homework and define what those requirements are first (and then spec out your needs with items available on the market) before assuming that a b/c some military/unit acquired it then that's the best there is (realize almost all military acquisitions are decision by committee with compromises that more often than not degrade the initial objectives of a given weapon system in the name of compromise/to meet requirements of chain of command staff who have never been in the field and budgetary folks both within the military and in gov't budgetary appropriators -- take a look at the F35 boondoggle for instance) -- b/c often decisions/configurations optimized for others won't be in line w/ your needs... Define your needs and purchase for your needs, don't copycat.
