Donaldjr1969 said:
Take some carbon fiber reinforced plastics and measure their tensile strength, shear strength, and other elasticity properties against steel. You will find they are very close to each other.
Just check out Young's Modulus figures for many materials, both metal and plastic.
Disclaimer: Not all of this post is directed at you, nor is it meant to be purely argumentative (that was just a happy side-effect), but rather intended to be informative.
Depending on what steel you're talking about, and what type/amount of carbon reinforcement you're talking about, this can be WILDLY incorrect.
Shear strength of carbon reinforced plastics is much lower than steel.
The tensile strength (in the direction of the fiber) of unidirectional carbon is significantly higher than steel, while the transverse strength will be much lower. The tensile strength of a quasi-isotropic laminate will be almost identical to a high carbon steel, and weight/strength will increase over that of steel.
For anyone wondering, this can be verified using matweb.com. It is a real-world database of properties of most structural materials.
Polymers and reinforced polymers are great. I love them, but they aren't ideal for everything. High impact-force density and high compressive stress areas will be better suited by metal in almost every case, while areas exposed to tension in a predictable direction are better suited by grained polymers and reinforced polymers.
As far as Young's modulus goes...
7075-T651 Aluminum: 10,400 ksi
General High-Carbon Steel: 2,000 - 34,100 ksi (depends on exact alloy and treatments)
Carbon-fiber reinforced acetal: 2,250 ksi
Carbon-fiber reinforced ABS: 3,400 ksi
Impact grade ABS: 203 - 406 ksi
Not all of those seem too terribly similar to me. :/
But, plenty of other very important properties are left out by these... many have been mentioned and over-generalized earlier in the thread. Impact strength, fatigue life, proper design with the selected materials, manufacturing defect allowance and tolerances, directional strength and so on and so forth.
That being said, I like polymer guns, and I like aluminum guns, wood guns, steel guns, and pretty much anything that shoots bullets. Nothing wrong with polymers imo, aluminum has it's purposes (and can have an incomprehensibly high fatigue life if properly designed, so saying steel's set fatigue strength is better is kind of a moot point), and steel... well, it's steel. Built our nation outta the stuff, it's a pretty nifty, extremely versatile and varied product.
Polymer is cheap, and as far as using it for the frame of a gun, plenty enough reliable for me and anyone else planning on less than 5 or 6 million +p rounds in the lifetime of the frame.