Well, this is something one can sink his/her teeth into and, well, er -- $$$$$$$ not pennies.
Why fix it if it is not wrong, I guess is a good question to begin with.
Parts changing can be trading a good working part for a poor one--if you don't know what you are searching for----seems I have done this thing before
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The relationship with a good smith should be cherished. One you can talk too. One that understands and will listen too what you are saying.
If it is a bulls eye gun it needs one thing---maybe if it is a game gun it needs something else.
Bushings are good things--if they are needed. Springs, firing pins, mag releases--sights-depending on the purpose.
Tight fitting--get a Les Bauer and be finished. Matter of fact I have almost come to the conclusion that one of these from Bauer, Brown, Wilson, etc can be less expensive--in the long run--than the build-up of a Colt, STI, etc . I guess that is dependent on your own personal bent and your own personal skill.
It is my own experience that there is hardly a drop-in part made and my experience is that my skill level is somewhere down on the making a spear point from flint and having too live off the results
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Blessings