Like any other malfunctioning firearm, complete disassembly and inspection of all of its parts is highly recommend. Then, thoroughly clean and lubricate the gun, which may be enough to fix the problem.
Many years ago, when I was a beat cop, I carried a Colt Mustang .380 ACP as my backup gun every day. A while back, when I was practicing at the range with this same gun, I noticed that once the pistol had a few rounds through it, the trigger got increasingly harder to pull and finally failed to fire at all after several magazines were run through it. Not Good!
Several years ago, the trigger broke, and I replaced it with a new one. Thinking this could be the problem, I replaced the trigger again to no avail.
Apparently, Colt no longer services these old pistols, so I need your recommendation as to what I might do to return this pistol back to proper working order.
D.P., Tucson, AZ
Without seeing the pistol, I can only speculate as to what might be causing your problem. Obviously, there is a friction buildup in the gun that manifests itself in an increasingly hard-to-pull trigger.
Since the Colt Mustang is carried cocked and locked, the back of the pistol where the hammer is located is open to the elements and all kinds of debris from blowing sand to pocket lint. The magazine well is open at its rear, exposing the mainspring and other moving parts to any kind of foreign material that may find its way into the gun as well. Unless the pistol received more than a simple fieldstrip for maintenance during its service life, it is likely that multiple contaminants have found their way into the inner workings.
My first inclination would be to detail strip the gun and remove any dirt, debris or old lubricant. Pay particular attention to any openings where springs are seated and parts are moving. Any material that didn’t leave the factory with the gun originally needs to go. Next, I would inspect every part for wear and deformation. Replace any suspect parts with new ones. Treat any normal wear surfaces with a quality lubricant. These are found by looking for the shiny spots where friction has polished the bearing area.
Once this is completed, reassemble the pistol and perform a function check. Check each magazine in and out of the gun individually to ensure one or more are not interfering with the movement of the trigger bar. Some magazines are slightly oversize, especially aftermarket varieties. There is a tendency for the left-front corner of the magazine to drag the trigger bar, causing a heavier feel to the trigger. The firing-pin safety, which is actuated by the ejector, should move freely in its recess as should the ejector. These areas are often overlooked when performing general maintenance.
Although fairly rare, a dirty chamber causing irregular seating of the chambered cartridge will cause the slide to not fully seat, resulting in a partial or full activation of the disconnector. Were this the case, a thorough cleaning of the chamber is imperative. When inspecting the chamber, ensure the cartridge seat at the forward edge of the chamber is sharp and without buildup. Coupled with being dirty and lacking lubrication, the recoil spring may be so weak as not to be able to fully close the slide, which can also result in activation of the disconnector.
All things considered; the service life of your Mustang is far from over. In my opinion, detail cleaning and lubrication along with replacing any worn-out parts—especially the springs—will return your Colt Mustang to service with the reliability you expected back when you and the pistol were both new on the job.