I used to use crush washers all the time too because they were easier until I talked to John at WOA and he explained how they were not the best choice. Now I use peel washers.
A smith vortex really doesn’t need one either. On a A2 bird cage FH, you need the ports to the sky and the flat part to the ground- the crush washer is put on first because the threading will not allign the devices- “crushing the washer” is use do align it- or peeling layers off the washer. Your SF556 may not need to be aligned.
Hope that makes sense I drank I lot last night.
For proper orientation of the muzzle devise…
and may I say I like the FCS556 also.
I gave up on peel washers (big hands…all thumbs).
I ordered an assortment of 1/2 ID…3/4 OD SS shims from Mcmaster’s
and made up my own kits like the KAC kits. Didn’t really spend a whole lot,
now I have enough for a lifetime.
I use them, but could be a pine in butt. You need to heat them up put in water and peel off untill you get it just right in time. the main reason for use of peel is so you can use a can if leg. in your state.