Top view of an X-Bolt 6.5 PRC magazine showing how the magazine will accommodate the 6.5 PRC with a 2.955" COAL.
To accommodate the SAAMI specification max overall length of 2.955" of the long 6.5 PRC cartridge, Browning made a few modifications to the short action length X-Bolt magazine. It should also be noted that the same general modifications have been made to the 300 PRC magazine as well except in a long action magazine.
The X-Bolt in 6.5 PRC is built on a short action rifle platform. There are great benefits with having a short action over a long action rifle (that's an article for another time) so Browning modified the magazine design to keep the 6.5 PRC in the short action length and take advantage of those benefits.
Two major modifications were made to the magazine. First, the front of the magazine has a deeper channel cut to allow for the longer length of the bullet tips. Second, with the channel cut is so close to the front of the magazine the latch spring has been moved from under the center of the latch being offset to one side.
Bottom view of an X-Bolt 6.5 PRC magazine showing how the latch spring has been moved to the side of the latch.
With these modifications, the 6.5 PRC cartridges fit nicely into the magazine with a little room to spare and feed ever so smoothly in the X-Bolt rifle.
My hand loaded 147 Gr. ELD-M bullets at the longest COAL I have loaded have been 2.9490" and they fit with room to spare in my magazine. See: 6.5 PRC Handloads