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This gun has a condition that makes any use unsafe prior to repair by a qualified technician.
True, you may very well have an accumulation of dirt and fossilized lube causing an issue, but since this may very well have been the issue that made g-pa put this up in the first place, you would be safety conscious if you had a proper gunsmith check out the parts condition and make a competent repair.
You are not able to be taught the intricacies of safe trigger diagnosis and repair without getting more than typical post material or advice.
Sooner or later, you need a gunsmith the same as you need a mechanic or physician.
I don't believe this rises to the level of dire emergency for you to be the one tackling this by symptomatic guesses from internet banter. This is not the same as being in a submarine performing an emergency appendectomy with a doctor on the radio and a medical book and sharpened spoon at the ready.
If you can't afford a proper repair by a gunsmith, you shouldn't be paying to use shells in a gun with issues pertaining to safety. Save your pennies to get that fixed and don't try to use this problem as your training ground of gunsmithing.
Choose a simple issue that won't have dire consequences from any amateur result on your part.
This condition might just be a disaster if your repair fails to be permanent, and if you can't tell if your repair is permanent, then subsequent use is just another test, right? And if it fails----?
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Anybody that has a gun that self-discharges is in no position to play with the safety of every other person within projectile range, and nobody should be recommending any amateur repairs of those issues without having some expert at least examine the situation firsthand.
Safe function is no accident, and free advice may be no safe solution- it's just cheap.
Kirby