Actually, springs can weaken with repetitive flexing. It depends on how close they are strained to their limits of elasticity.
Many springs do retain their spring rate over long periods of time with repetitive flexing. The springs in weight scales are a good example. But like engine valve springs, these springs start out with an uncompressed free length that is only marginally longer than their compressed length. So they operate well within their limits of elasticity.
Recoil and magazine springs start out with a free length that is much larger than their compressed length. So they operate much closer to their elastic limits. With lots and lots of use, these springs will noticeably weaken.
Spring rate is directly related to the coil wire cross sectional area, but it is inversely related to the number of coils; more coils, lower spring rate.
To maintain the same spring rate with heavier wire, you would need more coils.
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