Top 3 I have already done:
1. New pistol grip. Just don't like the standard grip angle. Excellent
2. LAR-15 10 round pistol mags. Necessary 1 5 round mag just doesn't cut it.
3. Red DOT optic. So much easier than iron sights. But you have done the optic already.
4. Change the front guard. Necessary for mounting things in my case.
What I would like to do:
1. Trigger. Stock trigger is not very good but a good trigger is $$$.
2. Trigger.
3. Trigger. In case you haven't gathered the worst part of my AR is the trigger. Just hard to justify the $'s. The existing is adequate but not much better than my Russian milsurps.
