This from JB's article in Rifle 295:
chambers are “proprietary Barrett designs to reduce bullet jump and improve accuracy,” according to Barrett’s website, and the rifing twists reflect today’s trend toward longer bullets with higher ballistic coeficients (BC). In the .243 Winchester, the twist is one turn in 7 inches (1:7), and since the short-action magazine box is 3 inches long, longer boat-tails can be seated out to the lands, even after the chamber throat starts to erode.
