Does keeping rounds stored in a magazine weaken the spring in the mag?
+1It is my understanding, and experience that what weakens springs are cycles. i.e. compression and decompression.
There have been experiences where someone had found a 1911 with fully loaded magazines over 30 years and function just fine..
Its the compression and decompression (i.e. loading and unloading) that wears out a spring.
I've had several M14 mags. loaded for years (10+) that functioned just fine when I used them.