There is a lot of hype about around various spring materials in the aftermarket firearms industry. Each company is trying to convince the buyer that they have found the 'magic' spring material and that anything else is unusable - such is the nature of hype.
Most of the common spring materials generally fall within a 10% window of maximum strength. This means that the fatigue limit (lifespan) of the various materials doesn't really vary all that much from one to another under most conditions. Physical design factors such as wire thickness, number of coils, and surface finish have a larger impact on the life of the spring than anything else.
The differences between materials really come into play when a spring has to operate in a demanding environment, such as elevated temperatures or in a corrosive atmosphere. For instance, recoil springs in weapons that produce a heavy volume of fire are often heated to well beyond the acceptable range for music wire. In such a case, one of the other materials is preferable.
The ideal material is the one that best meets your specific needs. As with most things in life, the more you learn about a subject the more you realize there is no simple answer. The most common materials used for firearms springs are listed below. Along with a description of each material, there is a short explanation of the advantages and disadvantages of each.
This is a general term referring to a range of high carbon steel wires. The term 'music wire' is used because guitar and piano strings are typically made from this type of material. It is the most common material for small springs.
Generally speaking, music wire springs are the highest strength springs for their size. Their high strength gives them excellent fatigue life which means they will last longer and perform better than most any other type of spring. Music wire also has a very high toughness and quite tolerant of shock loading. Music wire springs can be plated with relatively little difficulty, but platings do tend to cause some problems with manufacturing high tolerance springs.
Music wire is limited to low temperature applications. The material is not suitable for temperatures above 120C (250F) as it will loose strength. This material is also prone to rust and corrosion which can create very small imperfections in the surface of the wire. These imperfections can cause the spring to fail suddenly (break in two) under stress. This makes music wire a fine choice for competition springs or even home defense, but less desirable in the field where they are exposed to the elements.
This is an alloy steel. It has small amounts of both chromium and vanadium added to it to allow it to withstand higher operating temperatures. This material can be plated.
While not quite as strong as music wire springs they can be used for temperatures up to about 220C (428F) before is starts loosing strength. This material has a high shock load tolerance.
It has slightly less strength than music wire. It is also slightly less stress tolerant than chrome silicon; however, it is easier to use than so it is preferable where stress falls within acceptable limits. This material is also prone to rust and corrosion which can create very small imperfections in the surface of the wire. Again this makes chrome vanadium a fine choice for competition springs or even home defense, but less desirable in the field where is it is exposed to the elements.
Another alloy spring steel with typically just over 1% silicon added to increase heat tolerance. This material can be plated with proper techniques.
This is generally a preferable material to chrome vanadium as it is slightly stronger and can tolerate slightly higher temperatures, up to 250C. (482F). This material also has excellent shock loading tolerance.
Slightly less strength than music wire and like all the non-stainless steels this material is subject to rust and corrosion which can cause sudden failure of the spring. Again this makes silicon a fine choice for competition springs or even home defense, but less desirable in the field where they are exposed to the elements.
17-7 PH Stainless Steel:
This is a high strength cold drawn and precipitation hardened stainless steel. This material typically has a blue to brown color after heat treating. It has general-purpose corrosion resistance. It is magnetic in spring temper.
This material has elastic qualities similar to music wire while maintaining the corrosion resistant qualities similar to type 302 stainless. It is suitable for extended outdoor use, fresh water immersion, and moderate marine use. It is appropriate for use in temperatures up to 600°F (316°C).
It has slightly less strength than music wire. 17-7 PH is also relatively expensive and requires heat treating after coiling. While significantly corrosion resistant this material is not suitable for indefinite immersion in salt water.
Keeping it all in perspective:
In general, the greatest danger to guns springs is corrosion. Springs are typically the thinnest metal in the firearm and as such they are extremely vulnerable to catastrophic failure from corrosion. This is why when design parameters allow for it, the best choice for firearm springs that are expected to function in the field is a stainless steel.
The US military has been using stainless magazine springs for quite a while because they get the job done and survive being in the field better than any other material. When possible we select high grade stainless for our weapon springs for the same reasons. We are designing parts to survive a field environment.
In the end the material has to match the intended use. There is no single best material.