It's very niche, but works if you can't modify your rifle - FAB Defense Double Picatinny Rail for AR-15/M16/M4 Standard Handguard

I used these for some time, though only in training environments. In those environments, I was issued a stock M16A2 or M4 with the round handguards. These allowed me to mount a light on said firearms, which was a HUGE benefit, particularly when I reclassed to combat medic, and we had to find casualties in a dark building that was full of smoke. A railed or MLOK/Keymod handguard us vastly superior in every possible way, and there are good yet inexpensive handguards readily available, thar install with about the same effort, but if you are dealing with, say, a department-issued rifle, that you aren't allowed to replace parts on,this is a good option for a light, bur you can't use it for anything that requires a zero, like an offset dot or laser.