I am having a problem with a 39-2 slide locking back when there are rounds remaining the in magazine. I have three magazines with this pistol, one original (7 round) magazine and two other (8 round) magazines of unknown manufacture.

When I shoot the pistol, the slide release engages the slide and locks it back. This happens with just about any number of rounds in the magazine. It happens with all three magazines.

Does anyone have any idea what could be happening and how to fix it?

Perhaps the slide stop has a bad plunger spring.

Is it a "new" problem, that didn't happen previously?

Does it happen with someone else shooting it?

I had that happen with my new Kimber when I first shot it. I found out the magazine wasn't locked into place. I make sure the mag is locked in place now and it has not happened again.

I am unfamiliar with the 1st Gens but have several 3rd Gens of which one had a similar sounding trouble.

It's problem was twofold:

- the slide stop lever was closer to the frame than normal because of a wear mark on the frame moving the lever's arm (that contacts the magazine follower to lock back the slide) closer to the magazine by about ~.010".

- ammo using bullets with a squared off blunt nose would hit the lever's arm (especially if the ammo slid forward from recoil) as the next round sprung up to the top position locking the slide back even though it was not the last round.

Changing to bullets with a more pointed nose gave it a little more clearance & also allowed them of deflect any contact, which cured my problem 100%.

While my problem was no doubt unique to my setup it might be worth looking at these areas to see if the top round is hitting the slide stop lever's arm & causing your problem.

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Waiting for the break of day

My 1911 government did that. I reload 45acp and found the that the bullet's nose hit the lever's arm. When i measured the OAL of cartridge I found they were a tad too long. Ran them thru the seat die to seat the bullets a little deeper. Problem solved.

I have found that giving the stud against which the plunger in the slide stop rides, a slight turn so that the slide stop plunger has to go "up hill" to lock back usually will solve the problem. I have done this both on Models 39 and Models 59 to solve premature lock-back.

Be careful of moving that button, It's staked in place with a special tool. You do not want that loose at all.
Still waiting to hear if this is a"new" problem or not.

Not to worry. The amount of turn required usually is not great, and the aluminum "gives" enough to allow the turn without the stud becoming loose. I did this to several Models 39 and 59 back in the day and each one was fixed without the stud becoming loose.

My own Model 59 of 1974 vintage required this fix and it is still working without the stud having become loose. I am confident that such a fix will work for the OP's gun.

If paranoid about a loose stud, one could put a dimple in the stud aligned with the plunger, which should work as well. I prefer just turning the stud a bit (using a small adjustable wrench adjusted tight on the flat of the stud), which does not alter the appearance of the stud and cannot be detected.

This is not a new problem and has happened with multiple shooters, magazines and brands of ammunition.

Since this has been a long standing problem with this gun, and we have three magazines (one S&W, two unknown manuf.)

The rounds do not appear to hit the slide release.

The stud could be an issue. I'll have to take a look at that and see if it is loose and can be tightened.

Thank You for all of the suggestions!

I'm not on the computer that often, I've been busy with this and several other firearms. I will get back on when I've tried suggestions and/or found a solution.