This just start happening, while at the range firing my sd40ve (about 500rds so far) after about 4 or 5 rounds the slide stop engages as if the magazine is empty. so i rack the slide and it does it every 2 or 3 rounds. i have 4 magazines it only happens with 1 magazines which i have marked. this magazine is fairly new. is it just the springs are too tight or bad magazine? what do i do?

If it only happens with one mag, I'd guess that is the issue. How many rounds through this one magazine so far, since new?

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maybe about 75 not sure i've been using different mags

so can you fix a mag. she i switch the spring and follower and see if there is a difference

A suggestion to see why just one magazine has a problem.

Remove the magazine and remove the slide.

Then, just one by one, put empty magazines in and observe the action of the slide lock lever.

Of course they should ALL lift the lock lever but can you see a difference from one magazine to the next?

ok I'll try that thanks

also perhaps take the magazine apart and clean it , inspect for burrs

My 1911 did that. Found that the nose of the bullet was doing it. Solution was to seat the bullet a tad deeper.

Maybe just return that mag to where you got ?

Might be your grip. I don't know how you grip the pistol, but I had the same problem during my first 500 rnds. I found that placing my RH thumb OVER my LH thumb eliminated the issue. You may want to try this. As always, YMMV !!! Good luck !!