I have 2 recently acquired CS40 mags that have evidently been fully loaded since the 90's. They do not function properly compared to the other two I have. Sometimes the slide won't lock back with one. Load first round, second does not want to load as the first is getting hung on the lip of the metal. Sometimes the slide will not pick up a round. Very frustrating day at the range with these. Does anyone know a source for the springs? Wolff does not list them, nor does Brownell's, Numrich's etc.
Perhaps they could be stretched? That may be a good question for Fastbolt.

Based on your post, I looked for CS40 mag springs as replacements for my future use. Sure enough, I couldn't find any either.

I didn't note the availability of 1006 springs however, so I wonder if those could be cut down/modified for CS40 use.

Leaving magazines loaded for decades will not degrade quality springs.

Make sure the springs are your problem, swap the springs from the good mags into the bad mags and see if the problem moves.

What about 4013 springs,those are single stackers,Numrich lists those. CS40 and 4013 mags both have the 'back edge spacer' as they're used in 40S&W to 10mm conversion projects.

I took the CS40 to the range today and I was able to make one of the bad mags work part of the time. With the naked eye I can see no difference in outer appearance between the two mags that work and the two that don't. I thought of swapping the springs from the good mags also, and will do so soon and report back. I also plan to check to see if there is any difference in their lengths that is noticeable.

Had not thought of 4013 springs, I could remove a spring from one of my 4053 mags and see how they compare.

S&W parts list has the same magazine spring (part #209060000) listed for the CS40, 4013&4014 OBS, and the 4053&4054. You could call S&W and see if they have it/will have it, or Wolff also makes magazine springs for these models: https://www.gunsprings.com/index.cfm...D58/dID261#889.

Turns out not to be a spring issue. Both springs in the bad mags were just slightly shorter than the two in the good mags I have. So I swapped springs-same issue. I swapped followers-same issue. I put everything back and loaded one round in a bad mag and one in a good mag to compare.
The two mags that were bad would not allow the brass to go back as far as the good mags. A little adjustment to the guides on the inside of the bad mags and they are almost good as new. One works perfectly, the other will load correctly 90% of the time. Also, the one bad mag locks back on empty every time since. Thanks for all who replied with suggestions.

The one that works 90%, put a new spring in

I like new springs, you never know how a pistol was used before,
so why not take out any variables on that level?
Plus, you won't have to mess with springs again during your lifetime...

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Marlin & Calico Specialist