Laser and light go on the handguard. The more rigid the handguard system is the better laser accuracy. For the most part laser pointer is zeroed with constant offset and they are useful for close range applications.
When you're setting up your AR-15 with a laser device, you want everything to feel natural and easy. You shouldn't have to stretch too far to reach your tools or make uncomfortable grip placements.
Integrating a Laser
Most lasers should be mounted as far forward as possible and at the top rail (12 o'clock). This allows the laser to project forward without bouncing off a switch, sight, hand, or rail. It also makes it convenient to use the on-board buttons as your primary switch, or as a secondary if you want to use a remote switch.
Do not mount precision laser device on a handguard that can potentially deflect or on polymer (MLOK or KeyMod) rail attachment pieces.
Your setup should be ergonomic, comfortable, and easily accessible with pressure switches (also not interfere with sling attachment). Since everyone is unique, you'll need to experiment and adjust to find your perfect setup.
Keep an open mind, and make sure your setup works for both right- and left-handed activation, for the ways you plan to use your AR-15, and for your expected environments.
There are many ways to mount a light on your rifle, and several factors to consider. What kind of light are you using? Where do you want to mount it? Do you want a tail cap, pressure pad, or both? Also, what additional accessories will you be adding, such as lasers, sights, muzzle devices, and rail covers?
Where you mount your light is crucial. If you mount it on your rail, make sure you can activate the light from different body positions and with either hand. You also need to consider where you'll place your laser if you're using one.
A good rule of thumb is to mount your light where your hands naturally land on the rifle. Try to place it as far forward as possible without interfering with your suppressor or muzzle device. This helps avoid shadows and splash off the front of your rifle that may obscure your vision.
For different length rail systems, you will try different setups to find what you like. Above all, there are tons of light mounting accessories for MLOK, KeyMod and 1913 to customize the rifle the way you want.