Originally Posted By wrenchmonkey:
A KB is a KB, and it's not caused by upper receiver EVER. Period. If a weapon KBs, its because either the round was over-charged with powder, the barrel was somehow blocked, or the headspacing was WAAAY off, or some other problem with the bolt or barrel.
In any case, you can pretty much count on the upper (and lower) being trashed. How pretty it is afterward is irrelevant to the discussion, because ruined is ruined. Period. When I build a rifle, or reload rounds, or fire a rifle, I do what it takes to ensure that my rifle does not become a bomb.
If you're more worried about your rifle blowing up in your face, or how well your upper is going to contain the explosion when it blows up in your face, than how much it weighs, or how much it cost, or really much of anything else for that matter; my advice would be two fold:
1:) DON'T FIRE THE WEAPON, UNTIL YOU CAN TRUST THAT IT WON"T EXPLODE IN YOUR FACE.
2:) Perhaps consider finding a different hobby, besides rifle building.
I know you're a competent builder, as I followed your build of the pistol shown in your profile pic, and I respect your level of competence in that area; so I don't think you really have any need for concerns of your weapon exploding in your face. So I'm left to wonder what it is you're getting at.
When I ask for examples of failures, I'm talking about failures of the upper, caused by the upper. Not caused by a KB, or being run over by a tank, or dropped out of a jet, or placed in a furnace... Normal failures, caused by normal use (Even heavy or "extreme use", is fine by me). I really would be interested to see what level of use it takes to make these things fail, be it a lot, or a little.
I do agree in regard to wear surfaces. I'd like a metal wear surface too, based on what I'm traditionally used to. But unless we really establish how many rounds it takes to wear one out, we're really just speculating on what we think MIGHT happen. Will aluminum outlast today's polymners? Almost certainly. Will the rifle realistically ever be able to fire enough rounds to actually wear it out? Well, we don't really know that, and I would like to know. My guess is that a large majority of shooters will not fire enough rounds to wear it out, but again, that's speculation...
I've got to hurry up and finish paying off my taxes this summer, and then get to work on some business debt, but in the coming months, I fully plan on locating a polymer upper, and doing a little bit of a torture test...
I've wanted to experiment with a few of my own ideas about polymers, with regard to the AR platform for a few years now as well. So you may see something you haven't seen before, in the next year or two. ;)
I think if you are worrying about the KBs then think about this. Polymers are being used to reinforce or build explosive barriers in the middle east and high value targets across the world.
Polymers are more flexible than aluminum and will might absorb more of the explosion before completely loosing it. Aluminum is going to rupture and rupturing too soon is BAD.
Both AL and Polymers are going to rupture and will likely mess up the rifle if this happens. But there is a good chance the plastic will give a little before rupturing allow some energy to be dissipated before it tears apart. The harder the material the less it will give before rupturing. When it ruptures the least amount of energy left to dissipate is better. So if you are designing as an engineer, where you are designing to kill as few people as possible when it fails(because everything fails) then the polymer MIGHT, MIGHT, be the best bet.
Would I want a plastic AR? No. I don't even like plastic handguards or butt stocks. But weight is an issue. I own a FAL, so my next AR will be light.
EDIT: Editing out some words as I'm not 100% positive of what I speak, this is an opinion.