For AR-15 / M4 Carbine:

Made in the USA and 100% quality inspected by hand for proper function. 

Constructed from certified USA made chrome silicon spring wire

Heat treated, stress relieved

Provides consistent spring pressure for hundreds of thousands of cycles (estimated 750,000+)

Greater resistance to heat & high stress cyclic rates

Improves operation, long-lasting performance

Consistent pressure reduces bolt bounce

Significantly reduces the perceived "twang" noise which is common during firing

Link to parts manufacturer