The bolt and cocking system can be linked to the firing valve, as in blowback designs, or completely divorced from the valve, as in high end electropneumatic systems. As mentioned above, the bolt's operation needs a small regulator and pneumatic circuit if it is to be controlled independently of the marker firing. Bolt movement in relation to firing controls blowback up the feed tube, and causes a problem with all markers of the blowback design, and many community websites advocate a force-feed hopper and drilling holes in the feed neck to alleviate pressure before it travels into the hopper. In markers with a separate system to operate the bolt, the bolt is set to open at the precise moment where a vacuum is created in the breach, pulling a ball in. This was first discovered on Autocockers, and can be performed on any marker where the bolt can be left closed for a controllable amount of time after firing. Bolt fit into the barrel and breach is an important aspect of both velocity consistency and rate of fire.