I have a brand new SW1911 that I fired for the first time today.

I used 1 factory mag and 2 aftermarket (Wilson Combat) mags.

After the 3rd round ejects .... the slide stop engages the slide ever so slightly and it locks the slide open!

It does this with ALL 3 mags.

What can be the issue?

I would suspect that the end of the slide stop is mis-shaped where it contacts the plunger. If you have another 1911 try swapping the slide stops and see what the effect it. If you move the problem from one pistol to the other you'll be fairly sure of the cause. At that point I would recommend giving S&W a call. If they are smart they will send you a new slide stop and save themselves the expense of having you ship the pistol back to them to replace the slide stop.

OK. Thanks for the help.

What ammunition were you using?

Are you right-handed? If so, if you're skilled enough to shoot left-handed, try the same 3 magazines when shooting left-handed. If the "problem" doesn't occur shooting left-handed, re-examine your right-handed grip, with particular attention to your thumb placement.

If it continues to happen, you could call S&W to have it checked under warranty.

Typically, if "early slide stop" occurs while using different magazines, it's reasonable to look at the ammunition (bullet nose profile bumping against inside tab of slide stop lever), the shooter (grip, thumb placement) and then the gun (slide stop or plunger/plunger spring problem).

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Ret LE Firearms inst & armorer

Remove the slide and insert the slide stop in the frame and insert a loaded magazine and look to see if any part of the bullet is touching the slide stop lug. If it is you've found your problem. Wiggle the magazine forward and back and see if there is ANY contact with the stop's lug and the top round. The fix is to carefully stone or file on the part of the lug that is touching the top round. The other possibilities are the stop is bouncing when the gun is fired (very common when using an extended slide stop). A small dimple on the slide stop where the plunger presses when the stop is in the lowest position will fix that. Check to make sure that the plunger is traveling freely in the plunger tube. Sometimes a burr in there will keep the plunger from applying pressure on the stop or from even reaching the stop. Lastly many times this can be caused by the shooter's thumb hitting the stop when the gun recoils. This happens more than you would think depending on your grip and hand position. Occasionally you see a stop where the angle on the rear where the plunger presses on it is actually angled the wrong way which pushes the stop up all the time instead of pushing down on it. Good luck.