Once you have determined a location you can begin to set up for the recording. You may choose to utilize both close and ambient micing techniques.
Begin by placing a condenser microphone set to "omni" to the side of the gun (you will have to fire a few test shots to determine the best placement).
Adjust the microphone preamp and microphone to accommodate the high SPL produced by the gunshot (you may need to engage the pad on the microphone/preamp).
After you have adjusted the close micing setup, use some gaffing tape to place an "X" on the floor or ground to mark the spot. This is where the shooter will be positioned.
To add even more control over the direct sound, attach a lavalier microphone onto the gun itself.
Now we can begin to find the best location for the distant mic positioning. You may find it necessary to have two pairs of stereo mics positioned at varying distances.
Begin by using a pair of small "pencil" condensers and placing them in an X/Y pattern perhaps 20 feet from the gun and 6 to 8 feet up. This well help capture the overall sound of the gunshot.
If you are shooting indoors or near a canyon or mountain, try placing a stereo-spaced pair of large diaphragm condenser microphones facing the "back wall." This will capture the residual reverb that bounces off that barrier. This can later be blended with the closer mics to create more of an ambience with lots of decay.
The distance of the mics will be determined by how much space you will need to capture. I would begin by spacing them 6 to 8 feet apart pointer down, maybe 35 degrees. This will capture some of the early reflections from the reverberation.