Visually I could see no difference between the good and bad mag. So, I stretched the spring. It failed to lock back once out of 6-7 tries. So, I think that's an improvement. Thanks.
Having spoken to my fair share of gun makers (as an armorer trained by a few of them), and at least one major spring maker (who makes springs for aftermarket sales and some OEM applications), stretching a suspected weakened mag spring usually isn't a good idea. It can further weaken an already weakened/worn spring. When it comes to magazine springs,
stretching one that's suspected of having become weakened is usually only observed to be a short-term "fix", meaning it will soon revert back to its previous weakened state, and might become even worse. Replaced of the weakened spring with a new one is the preferred correction and repair.
Now, magazines are
assemblies, meaning they're made of various parts, so it's prudent to inspect the condition of the various parts when cleaning them, as well as when replacing any particular parts.
If you've fired "several thousand rounds" through that little gun, and the bulk of them were fired using that mag, it's not surprising the mag spring might be reaching the end of its service life. Failures-to-lock-back the slide can be a hint that a spring is reaching the end of its useful service life. Next up might be failures-to-feed, especially toward the end of the magazine load (when the spring is under the least amount of tension).
Some gun makers suggest armorers consider replacing wearable parts like recoil and mag springs on some time/use interval. One company recommended replacement every 5 years or 5 thousand rounds fired ... another gun company recommended replacing recoil springs every 2000, 2500, 3000 or 5-6000 rounds (depending on model/caliber), and the mag springs at least with every other recoil spring replacement,
or whenever the slide wouldn't lock-back during an inspection,
or whenever range performance indicated a weakened spring (which kinda covers all bases and contingencies, huh?
). Others didn't offer specific recommendations, but offered to inspect the springs and pistol functioning and evaluate when replacement was needed, etc.
I remember one gun maker once used "free length" comparisons of used versus new/fresh springs. Another one talked about examining the length of coils sticking out the bottom of a magazine when the floorplate was removed, but the follower and the top of the spring were still at the top of the mag.
The thing is that I've seen batches of brand new springs that differed by a coil's worth of free length even when brand new and never installed in a gun or magazine. Manufacturing tolerances.
Then, there's also the consideration that mag and recoil springs are relatively inexpensive parts to make, and when a spring maker ships a case of 10,000 springs, it's not like each of them have been tested for tension. Some might be at the low end of the expected tolerance range, and some at the higher end. Also, if there's any unrecognized defect in the material or heat-treat, a particular spring might have less of a service life than normally expected.
OP, you might call SIG customer service and explain your problem to them and see what they say. Don't be surprised if they want a credit card number to send you replacement mag springs, since you've fired several thousand rounds through the gun and springs are a wearable part. You might also ask about a new RSA (recoil spring assembly), while you're at it.
