Had a colt small pin sp1 lower for many years. About 5 years ago it double tapped on me. Then it started to happen occasionally. Yesterday i had it out and it was doing it every mag. I started parts swapping. I tried three different fire control groups in it. All of them seemed to be acting normal when it was cocked by had and dry fired and the problem only happened when it had an upper on it. Tried a different bolt and carrier out of another gun thinking maybe the firing pin was sticking. No change. Even tried swapping out the saftey selector. The upper i had on it was a new one id never shot before so i figured there might be a problem with that so i swapped another upper on it no change. By now i was getting nervous and didnt want the law coming by so i stripped the lower down to bare and tossed it in the garbage. So now i need another new bare lower. Cant for the life of me figure out whats going on. Ive had that lower for 30 years and only in the last 5 years has it started this. Then i went on psa (who has the only bare lowers i can find) and found what was 45 bucks a year ago is now 95 bucks!! What was happening is it would go bang when i pulled the trigger and again when i released it like a binary trigger would. Scratching my head on this one. Isnt my first rodeo. Ive built at least a dozen ars.