Some older FH designs like the SP1 on a M16A1 use a split washer merely to avoid it backing off, they were soft, were flat without burrs and the correct inside and outside diameter(1/2"ID, 3/4OD), didn't index, not concentric, and stood a good chance to damage the threads and face of the barrel. Using a random hardware store washer stands a much better chance of damaging the FH, maybe even the barrel or muzzle, they are unknown hardness, coatings or metallurgy can damage finish, rust or corrode to the barrel, probably the wrong size even if 1/2", and nowhere near flat or concentric, I doubt a reputable gunsmith would recommend ANY part with such a wide variety of tolerences and such a high risk of damage. Best is machining to index at proper torque without a washer, some designs use locknuts, but a shim kit or peel washer will at least index it properly and concentric. A crush washer is easy, correct dimentions, relatively safe, and can index a brake, but they don't crush evenly, and can leave the device off axis, no big deal with a short brake or FH, but could cause baffle strikes in a suppressor or hit a long tight brake.