I have a preban Colt 6520 and I'm going to buy a Colt M4 flattop upper. I basically have two choices of uppers a 16" 1:9 twist or a 14.5" 1:7 twist which will need a flash suppressor permanently attached to meet the legal length limit. I'm going to top it of with a Trijicon TA01NSN. I'm concerned that the bullet drop compensator in the TA01NSN is tuned for a 62gr bullet out of a 14.5” 1:7 twist barrel and I don’t know how different the 16” 1:9 twist will be. I would like to use IMI 855 as my standard ammo. So what do you guys think is best in this situation. FYI, even though I have read about the barrel twist in different AR-15 FAQs I’m still a little fuzzy on the differences.
Thanks for your help,