Setting up a white light on a long gun can be somewhat frustrating in that the device can take up a good bit of rail space, the activation control can end up in an awkward-to-reach location and mounting fittings can become complicated. Here are a few tips to make the process easier:
You'll need to "map out" the setup before buying the parts. If possible, try to position the light on the side of the gun that makes most sense for its particular method of activation by your support hand thumb or fingers. If you prefer to always activate long gun lights with the same digit, and you have multiple, different, platforms on which you wish to mount lights, strive to retain that consistency so that your "muscle memory" for that task transfers from platform to platform. For instance if you're a right-hander and you mount a light on your AR's right-hand fore-end rail, you'll have to run a tape switch to the left-hand rail to activate it if you want to do so with your support-hand thumb.
If you're mounting a Grip Pod, however, you may be able to use that company's factory rail accessory to mount a light on the unit's right side and simply bump the tailcap switch with your left-hand thumb since it will naturally fall in that area when that hand is in position on the Grip Pod's vertical section.
Regardless of how you do it, the simpler you can keep the installation the better. The more Velcro, wires and switches there are, the more things can go wrong when gear starts getting dragged across the ground or through bushes. Some so-called "weapon lights" have built-in Picatinny mounts. Otherwise, any quality flashlight that can stand up to recoil and that has a 1" diameter body can be mounted with a simple riflescope ring. Wire ties or gaffer's tape can help to secure loose wires for tailcap switches. Just don't forget to keep a few spare batteries with the gun.
What suggestions do you have for mounting lights on long guns?