Problem
Hi Clark, and welcome aboard.
A fairly common problem, and one that's usually pretty simple to cure.
Oddly, it seems to happen most often with hardball.
Possble causes:
Bullet nose nudging the slidestop lug as it rises in the magazine, and causing it to engage. Slide off...Slidestop installed...with a good light, load rounds into the magazine, and move the mag up and sown in the well and look for contact. Use a small, flat file to relieve at that point, and follow the existing angle as closely as you can. It doesn't take much clearance...maybe .010 inch maximum.
Weak plunger spring or worn pin. Look at the pin to see that the end is still nicely radiused and not flattened or beveled. Remove the plunger assembly and stretch it a little at the dogleg and try it. If that cures it, get a new spring. If it doesn't, the stretch may last a long time before collapsing, and it may not. Complete assemblies are available through Brownells in the Colt factory section for about 5 bucks. A good idea to keep a spare or two on hand.
Incorrectly machined slidestop. Try a different stop to see if te problem goes away. In fact, this probably should be your first step before doing anything else.
You may be unconciously nudging it with your thumb during recoil. If you shoot with both thumbs laid alongside the gun, pointing at the target, this is a real possibility. I've had guys bring their pistols to me at the range with the same problem, and when I fired it...the bug didn't bite. Shoot the gun again, and be especially careful not to let your thumb touch. Firing one-handed is the best test for this.
