I recently acquired a Colt Sporter HBAR AR15 with 20" barrel, standard A2 configuration. When I brought it home, I broke it down to clean it and noticed that the bolt carrier is what I believe is called the half-circle variety. I have never seen one of these b4. Will a standard closed bottom bolt carrier fit in my Colt? Is there any advantage(s)/disadvantage(s) to the one supplied with my rifle as opposed to the closed bottom bolt carriers that I am used to seeing? Any useful information is appreciated. Thanks.