No, this will only fit picatinny.
This will fit anything with a Picatinny rail
The FAB Defense Quick Release Vertical Foregrip mounts to picatinny rails. So if the Boss 25 Shotgun as a picatinny rail available to mount the FAB Defense Quick Release Vertical Foregrip it will work.
This FAB Defense Quick Release Vertical Foregrip could fit a shotgun. It all depends on what kind of shotgun and how it is set up.
In order to attach the FAB Defense Quick Release Vertical Foregrip, you would need to replace your handguard with something that uses picatinny rails. At that point you can mount this foregrip or any other that mounts via picatinny. I will link a possible handguard for you below:
https://www.opticsplanet.com/leapers-utg-pro-made-in-usa-model-4-carbine-length-tactical-quad-rails.html
Yes. The FAB Defense Quick Release Vertical Foregrip will fit to any picatinny rail.
Yes. As long as your Scar has a Picatinny/1913 rail on the handguard you wont have an issue fitting the FAB Defense Quick Release Vertical Foregrip.
If you have a carbine length gas system, I would recommend the Midwest Industries 2 piece handguard found here:
https://www.opticsplanet.com/midwest-industries-gen2-two-piece-free-float-handguard-extended-length-carbine-b.html?_iv_gas-system-length=carbine-length
Midwest Industries also offers handguards for mid length, rifle length, and extended length gas systems.
Yes, this will attach to the bottom Picatinny rail
You will need an M-lok Picatinny adapter found here.
https://www.opticsplanet.com/presma-3-piece-picatinny-rail-section-kit-for-m-lok-style-slots.html?_iv_code=2HC-T8-PP3ML-RLMLK1