PSE Tac nocks are glue in for a very specific reason. The shaft is a weave shaft and it gives in many ways, some gives more than others. If the nock is not glue in (like Firenock D) when there is pressure from shooting and the specific shaft gives radically (i.e., the diameter of the shaft is allow to expand) the nock can be forced into the shaft. In case of the D nock, the battery or the circuit board can be crushed. This situation is observed not only with Firenock D but also with factory black nock that is not glue in. Many who has seen this and was told one need to glue the nock and be done. The gluing of the nock is not a good answer for that means one must go back to the nock that cannot lit while most PSE owner agrees that they need a lighted nock to see where the arrow goes with such a fast bow. To address this, the solution to this issue is to look at the real problem/issue here. It is not the nock that is at fault, but the shaft. Its design allow it to flex radically, so we need to control that so the shaft would not expand radically. Some research later, I believe I have the perfect fix! The outside diameter of the Tac-15 is 0.350". Carbon express CXL™ Pro 250 's outside diameter is 0.350" and it has a BullDog™ nock collar on it. The collar hold the back end of the shaft from expanding. I believe this is the perfect answer for controlling the Tac shaft from expanding. Like all Firenock application with bull dog collar, it is imperative that one must glue the collar onto the shaft or the Firenock will be decapitated when shot into hard object due to the movement of the collar. It is better to install the collar first if you decide to use it. The collar added a full 1 mm to the length of the shaft. If the extreme shock end cap was installed first then the bull dog collar was added later, the depth of the extreme shock end cap would be 1 mm too deep which would allow the circuit board room to move! At 405 fps impact, it can spell disaster! If you already installed the extreme shock end cap, make sure you added a tiny piece of paper to the bottom of the battery or anything that you can add to pick up that space so NOTHING moves! Case in point, my customer Rodger Peck just send me the picture below. Please make sure your bolt is the latest as they do feel tighter with Firenock D than the one PSE make 1.5 year ago. If you remotely feel that the nock is not tight on the shaft, using the Carbon Express CXL bulldog collar as discussion above. Installing the bull dog collar on the back of the shaft will guarantee to eliminate the possibility of the nock being push into the shaft issue. Please be careful and take all precaution. Please also note that if the nock is not totally sit flush with the shaft this can also happen. After discussion with PSE regional manager, it is confirm that he is able to reproduce this when the Firenock D is not totally sit flush on the shaft. i.e. a little gap between the nock and the shaft will also cause this.