When Stringer Bell is shot the camera angle behind the staircase shows that his body falls in such a manner that only his legs are showing from that angle. When his killers are leaving the said camera angle shows that his entire body can now be seen.
Brother Mousone admiringly notices that Omar is carrying a .45 1911, but 1911 pistols are available in a variety of chamberings. The most famous and common is .45ACP, as it turns out that Omar is carrying, but at that distance and in that low light, there would be no way that Brother Mousone could know which round Omar's 1911 was chambered in until he opened fire and heard the report.