I have an old colt sporter that I put a heavy 20" barrel on and shoot in competition. problem is that very occasionally it doubles- maybe once in 500 rounds. I bought it used so I don't know what was done before to it, but inspection doesn't show abuse. oddly, it has a single stage,very nice trigger, but the parts seem to be stock.
inspection reveals wear on hammer notch (where disconnector picks it up). my guess is worn hammer notch, weak disconnector spring, worn disconnector, or combination. pins don't seem to have any play.
I'm inclined to replace the hammer and maybe disconnector spring.
it never does more than a double.
Advice? am I missing something? thanks.
i'd like to be sure i get this solved since my wife will be using this rifle in a two day clinic in may.

i am a usmc armorer i get doubles all the time i try the hammer first if that dont works then the semi disc. that almost always fixes it.

One in 500 shots? Are you using a light, competition trigger? Not having a firm enough hold on the stock can produce this as well. I've done it and seen it happen to other shooters.

Does your rifle pass the disconnector test?

Ensure rifle is EMPTY and while pointing muzzle in a SAFE direction, and safety selector on FIRE: retract charging handle, dry fire rife - HOLD TRIGGER DOWN - while pulling the charging handle again, release
the trigger there should be a dull "click" as the trigger resets itself but the hammer should NOT fall.

If it does - you have a disconnector problem.

I have seen a bad disconnector on a brand new Bushmaster Varminter with Bushmaster competition trigger and the owner of that gun was getting doubles every time. Not just once in 500 shots.