there is 2 different diameter pins. obviously the larger will not fit in the smaller receiver hole,but the smaller pin will fit into the larger receiver hole. and fall right out. what is most likely the problem is the spring in the trigger group that is around the hole for the pin, is stretched or broken. you can get replacement trigger group pins that "don't walk out". removing,installing, removing, installing. repeatedly will loosen the springs or break them.
Scattergun Bob would most likely know more on this trouble. he's most likely torn apart more that i can count on my fingers and my toes
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